SEM
based electro‐optical characterization of core‐shell
LED
s
and simulation of imaging including
CL
and
EBIC
excitation inside ensembles
Authors: Ledig Johannes , Frase Carl Georg , Steib Frederik , Hartmann Jana , Wehmann Hergo‐Heinrich , Waag Andreas 
Published: December 2016

Three dimensional (3D) nano‐ and microstructures (NAMs) are attracting a lot of attention and are discussed regarding several applications, especially in optoelectronics and sensors. For example GaN based 3D light emitting diodes (LEDs) with a core‐shell geometry are supposed to have substantial advantages over conventional planar LEDs: The active area along the sidewalls of hexagonal GaN pillars can considerably be increased by high aspect ratios ‐ leading to a lower current density inside the InGaN quantum well (QW) at the same operation current per substrate area.
[
1
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Thus related methods are requested for characterization of local electro‐optical properties with a high spatial resolution on single structures as well as in ensembles. Usually, electron microscopy is employed to investigate the geometry and properties of such 3D‐NAMs and for mapping of vertical features by an SEM a certain sample tilt (e.g. about 30°) is needed. Investigation of single 3D‐LEDs by electron beam induced current (EBIC) using an SEM based manipulator setup proves the presence of a pn‐junction and doping type of the core and shell, while cathodoluminescence (CL) gives an insight to the optical properties of the QW
[
2
]
. But in contrast to SEM on planar regions the interactions of the electron probe are significantly affected by the 3D geometry and the surrounding of the NAMs.

In ensembles of 3D‐NAMs a certain portion of incident electrons are scattered into neighbor structures and conventional SEM signals (SE, BSE, CL, X‐ray emission) are partly shadowed. This interaction is affecting the SEM imaging contrast and the probed signal also includes contributions which are not related to the material properties at the electron beam spot. As such parasitic signals are generated quite close to the original region of the interaction most (global) SEM detectors cannot separate them from the original source. In particular scattering events occur in an enlarged volume of the sample (of the substrate and NAMs) leading to a reduced excitation density and parasitic effects, e.g. this causes a significant contribution of defect related yellow luminescence (YL)

We present results of InGaN/GaN core‐shell LEDs obtained with an FE‐SEM which is equipped with SE, In‐Beam SE, low‐kV BSE, EBIC and monochromatic CL detection as well as a piezo controlled manipulator setup, see Figure 1. A modified parabolic collection mirror enables measuring luminescence from planar samples up to 4″ in a tilted view up to 30°. For a quantitative interpretation of CL and EBIC measurement values and image contrasts, the physical modeling of SEM images and spatially resolved energy transfer by a probe spot is necessary. This is performed using the simulation program MCSEM
[3]
. It models the different stages of image formation and generates SEM images of complex NAM shapes using e.g. GaN as model material. Aspects of the simulation are the electron probe formation, a 3D model of the specimen structure, the interaction of electron probe and solid state by means of scattering trajectories, the emission of secondary electrons, and different types of electron detectors, see Figure 2 and Figure 3. An insight to CL and EBIC imaging is gained by evaluating the scattering energy deposited in a distinct volume inside the NAMs as an imaging signal ‐ this is related to the generation rate of electron‐hole pairs inside the respective volume of the semiconductor.

Consent to the experiments this simulation reveals an edge contrast and shadowing of signals by the ensemble as well as scattering of primary electrons inside the ensemble of 3D‐NAMs. A quantitative comparison is possible by the absorbed current (EBAC). Artefacts of the EBIC are also demonstrated by the simulation, in particular edge contrast by a reduced generation rate and parasitic signals by scattering from neighbor structures.