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ELECTRON MICROSCOPY Fig. 3: Stereographic SE image generated by tilting the beam by ±0.2° on a 24 µm field of view of an ensemble of InGaN/GaN core-shell LEDs obtained with 10keV and at a sample tilt of 30°; one structure is contacted by a tungsten probe tip. The anaglyph can be viewed by red-cyan glasses. Fig. 2: (a) Spectra of CL and EL obtained by exciting small areas at the top part of the sidewall by a 590 pA electron beam of 10 keV and by driving a current of 1.5 µA from the probe tip contact to a buffer contact, respectively. (b) CL mapping of the InGaN MQW emission using a 500 nm short pass filter. emission (NBE) from the GaN outside the active region. Tilting the Electron Beam The electron optics (EO) of this FE-SEM is also capable of rocking the electron beam in a cone of up to ±12° for mapping of electron channeling patterns (ECP) on a small area of about 15 µm in diameter. Such ECP are also used to align the sample (via stage tilt and rotation) in specific diffraction conditions of the crystal structure to evaluate the density of threading dislocations and its type by electron channeling contrast imaging (ECCI) [6]. By switching from the BSE detector to the mirror for light collection also a correlative analysis of ECCI with EBIC and CL can be performed at the same diffraction condition. This EO tilt can also be used to image the sample by the SEM from a certain direction without affecting the tip contact by stage movement. A subsequent scanning of the sample from different incident directions enables topography reconstruction and generates a three dimensional impression, e.g. by a stereographic image as given in figure 3. Beam Blanking for Electrical Measurements Beside in situ measurements the point contacts are used to obtain the IV-characteristics and also electroluminescence (EL) utilizing the CL-setup while blanking the electron beam (fig. 2). Care is given to arrange the probe tip for contacting in the position of the optical focus, neither touching nor significantly shadowing the mirror and sample. Due to lack of a contact layer the current is crowding locally at the contact [5]. EL spectra evolve already at currents of a few nA locally driven through the point contacts on small InGaN/GaN LED structures. The MQW emission of EL is shifted compared to CL which is assigned to the different types of excitation and might also be related to an inhomogeneous MQW stack. No significant current spreading occurs along the p-type GaN shell as it has a lower conductivity than the n-type region; hence the EL is mainly originating from a small volume close to the contact. Subsequent contacting at different positions therefore gives a sub-µm spatial resolution of local electro-optical properties and the gradient of the InGaN composition along the sidewall facet can also be revealed by EL spectra [1,7,8]. Acknowledgements The financial support of the MWK, DFG and BMBF is highly acknowledged. References All references are available online: http://bit.ly/IM-Ledig Affiliation Braunschweig University of Technology, Institute of Semiconductor Technology and Laboratory for Emerging Nanometrology, Braunschweig, Germany Contact Dipl.-Ing. Johannes Ledig Braunschweig University of Technology Institute of Semiconductor Technology, epitaxy competence center ec² and Laboratory for Emerging Nanometrology Braunschweig, Germany j.ledig@tu-braunschweig.de www.tu-braunschweig.de/iht Read more about microscopy of light emitting diodes: http://bit.ly/IM-LED Further information on cathodoluminescence microscopy: http://bit.ly/CL-SEM [1] All references: http://bit.ly/IM-Ledig 46 • G.I.T. Imaging & Microscopy 2/2016
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