15. Nov. 2012, 16:15 Uhr, Gebäude NW1, Raum H3
Nanocharakterisierung von Halbleitern mit Heliumtemperatur-Kathodolumineszenz im Transmissions- elektronenmikroskop
Prof. Dr. Jürgen Christen, Universität Magdeburg
For a detailed understanding of complex semiconductor heterostructures and the physics of devices based on them, a systematic determination and correlation of the structural, chemical, electronic, and optical properties on a nanometer scale is essential. Luminescence techniques belong to the most sensitive, non-destructive methods of semiconductor research. The combination of luminescence spectroscopy – in particular at liquid He temperatures - with the high spatial resolution of a scanning transmission electron microscopy as realized by the technique of low temperature scanning transmission electron microscopy cathodoluminescence microscopy (STEM-CL), provides a unique, extremely powerful tool for the optical nano-characterization of
semiconductors, their heterostructures as well as their interfaces.
Our CL-detection unit is integrated in a FEI STEM Tecnai F20 equipped with a
liquid helium stage (T = 10 K / 300 K) and a light collecting parabolic mirror.
Panchromatic as well as spectrally resolved (grating monochromator) CL imaging is used. In CL-imaging mode the CL-intensity is collected simultaneously to the STEM signal - typically chemical sensitive HAADF Z-contrast - at each pixel. The TEM acceleration voltage is optimized to minimize sample damage and prevent luminescence degeneration under electron beam excitation. In CL-spectral imaging mode, a complete CL spectrum is recorded at every single pixel.
Subsequently, by evaluating the complex data matrix , sets of
simultaneously recorded monochromatic mappings CL peak
wavelength mappings CL(x,y), local spectrum linescans, local CL spectra, etc.
can be processed.
Typical results which will be presented include nm-scale correlation of the optical properties and strength and appearance of structural defects in: strain engineering AlN interlayers in GaN-on-Si structures, lattice matched AlInN/GaN distributed Bragg reflectors (DBRs) and complete vertical cavity surface emitting laser VCSEL structures with all-epitaxial as well as hybrid DBRs for blue (InGaN QWL cavity) and UV (GaN cavity) emission.
Minority carrier diffusion lengths of < 17 nm on one hand, as well as the efficient carrier transfer over > 150 nm into the quantum wells are directly measured in STEM-CL linescans. The impact of structural defects like dislocations is directly visible. The individual layers of the epitaxial DBRs and the nanoscale properties of their mutual interfaces are clearly resolved.