Exciton-polaritons in semiconductor microcavities are ideal for the study of the exciton-light interaction and its dependence on light polarization. In this work, we report on the optical response and the dependence on polarization of a polariton microcavity using microreflectance anisotropy spectroscopy (μ-RAS) with a spatial resolution of 10.0 × 10.0 μm 2. We have found that, in contrast to optical reflection, the μ-RAS spectra are quite inhomogeneous along the microcavity surface. We demonstrate the existence of microscopic local domains with differences in optical anisotropy of up to 20% within 100 μm. These variations are independent of the detuning between the optical and excitonic resonances, which in our sample is close to 0 meV. The μ-RAS line shape can be understood by using a model based on the anisotropic strain fields induced at the interfaces of the microcavity. The model agrees quite well with the experimental results and allows us to quantify the split of the energy levels of the exciton-polariton branches induced by the local break of symmetry at the microcavity interfaces.
We report reflectance (R) and reflectance difference spectroscopy (RDS) spectra of wurtzite heterostructures grown on Si(111) and Si(110) substrates in the energy range from 2.0 to 3.5 eV. Due to the threefold symmetry of the Si(111) surface, the heterostructures grown on this surface will relax isotropically through the formation of misfit dislocations, preferably at the AlN/Si interface, and no in-plane anisotropies are expected. In fact, only a small in-plane anisotropy of reflectance is observed, due to the initial residual off-cut of the silicon substrate that leads to wurtzite layers with a c-axis slightly tilted with respect to the surface. In contrast, for the Si(110) substrate, strong differences in lattice parameters appear between silicon and GaN/AlN depending on the considered in-plane direction, leading to a large in-plane anisotropy. By using a multiple reflection model for R and the in-plane anisotropies described, we developed a model to describe the RDS spectra in the vicinity and below the fundamental gap of GaN.
We report on photoreflectance anisotropy (PRA) spectroscopy of InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQWs) with extremely thin coupling AlAs barriers grown by molecular beam epitaxy (MBE), with no intentional doping. By probing the in-plane interfacial optical anisotropies (OAs), it is shown that PRA spectroscopy has the ability to detect and distinguish semiconductor layers with quantum dimensions, as the anisotropic photoreflectance (PR) signal stems entirely from buried quantum wells (QWs). In order to account for the experimental PRA spectra, a theoretical model at k = 0, based on a linear electro-optic effect through a piezoelectric shear strain, has been employed to quantify the internal electric fields across the QWs. The dimensionalities of the PR lineshapes were tested by using reciprocal (Fourier) space analysis. Such a complementary test is used in order to correctly employ the PRA model developed here.
The integration of zincblende semiconductors on silicon demands for a real-time control of the crucial steps of epitaxial growth process at a microscopic level. Optical probes, being non-invasive, are very useful in monitoring such processes at a microscopic level. By using the reflectance anisotropy technique with microscopic resolution (μ-RD/RA), which detects the difference in reflectance for two orthogonal crystal directions, we measured the optical anisotropies below and above band gap of orientation-patterned GaP structures deposited on both Si(100) and GaP(100) vicinal substrates. We have developed a physical model to describe the line shape of the spectra below and above the fundamental gap of GaP. By using this model, we have successfully analyzed μ-RD/RA spectra, and we were able to do anisotropy topographic maps of the surface and buried interface, which are consistent to those measured with scanning electron microscopy.
Micro reflectance difference spectroscopy (m-RDS) is a promising tool for the in-situ and ex-situ characterization of semiconductors surfaces and interfaces. We discuss and compare two different approaches used to measure m-RD spectra. One is based on a charge-coupled device (CCD) camera, while the other uses a laser and a XY translation stage. To show the performance of these systems, we have measured surface optical anisotropies of GaSb(001) sample on which anisotropic strains have been generated by preferential mechanical polishing along [110] and ½110 directions. The spectrometers are complementary and the selection of one of them depends on the sample to be investigated and on experimental conditions.
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