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Optica Publishing Group
  • Quantum Electronics and Laser Science Conference
  • OSA Technical Digest (Optica Publishing Group, 1999),
  • paper QThJ5

Second harmonic spectroscopy of semiconductor nanostructures

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Abstract

Semiconductor nanostructures and their application to optoelectronic devices1 have attracted much attention recently. Lowerdimensional structures, and in particular quantum dots, are highly anisotropic resulting in broken symmetry as compared to their bulk counterparts. This is not only reflected in highly anisotropic linear polarization properties, as studied recently in pyramide-shaped self-assembled InGaAs quantum dots,2 but also in second harmonic generation (SHG), which can be greatly enhanced allowing for detailed studies of such structures.3 SHG has contributed considerably as a technique to investigate solid state systems where the local inversion symmetry is broken by e.g. a surface or an interface, defect states or simply by structures so small that the bulk symmetry properties no longer are valid.4 Our idea is to use SHG in the configurations, where the bulk and surface contributions are forbidden for a homogeneous sample, so that the only source of SHG is associated with nanostructures embedded in the host material. Our first measurements in GaAS showed that indeed the SHG in reflection at normal incidence from a (001)-substrate, where SHG from bulk and surface is forbidden by symmetry, was more than 3 orders of magnitude weaker than that from a (11l)-substrate. We can therefore study various nanostructures embedded in the GaAs host material. In this work we focus on SHG from an MBE-grown sample consisting of 20 In0.5Ga05aslayers spaced by 5 nm GaAs layers. The InGaAs layers are grown to a Stranski-Krastranow phase transition resulting in the formation of pyramide-shaped self-assembled quantum dots with base of 18 nm and height of 5 nm laterally spaced by 55 nm on average on top of a wetting layer.2 The HeNe excited luminescence spectra at room temperature have the quantum dot states as the main spectral component. However, both the GaAs spacer layers and InGaAs wetting layers are observed as shoulders in the spectra, see Fig. 1. In Fig. 2 we show our first results of SHG from this type of semiconductor structure. At a fundamental wavelength of 900 nm the SHG signal is linearly dependent on the number of InGaAs layers that may be associated with the vertical coupling of quantum dots,2 see inset of Fig. 2. Moreover, measuring the SHG at different wavelengths we can identify both the GaAs spacer layers and the InGaAs wetting layers, see Fig. 2. Both layers are expected to have rather weak SHG generation, however, the formation of the quantum dots in between the two layers break the symmetry and gives SHG. The peak at 856 nm from the GaAs spacer layers are attributed to the enhanced SHG from the thin quantum well, whereas the peak at 880 nm is attributed to the enhanced (due to the nanostructure) SHG from the GaAs substrate. The wider SHG resonance reflects the InGaAs wetting layers. These results show that indeed we can use SHG for spectroscopy of semiconductor nanostructures, as the SHG signal from a pure GaAs sample is very weak and unstructured as expected. Our current work focusses on measuring not only the quantum dot SHG, but also further on the symmetry properties as they will be reflected in the SHG. Another avenue for us will be to use SHG to obtain high spatial resolution in the experiments5 and avoid spectral blurring from the inhomogeneous distribution of electronic states, that can be an obstacle for obtaining accurate information on these important nanostructures.

© 1999 Optical Society of America

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