SEM can be used to characterize the crystal structure at smooth surfaces, e.g. by mapping of electron channeling pattern (ECP). Layers of GaN grown on foreign substrates usually include a huge amount of threading dislocations (TDs). ECP are also used to align a sample in specific diffraction conditions of the crystal structure for evaluating the density of TDs and its type by electron channeling contrast imaging (ECCI)¹.

Beside columnar rods ^2,3 also elongated µm-structures like fins ⁴ with high aspect ratios are supposed to have substantial advantages over conventional planar optoelectronic and sensing devices. Thus the synthesis of such 3D-structures, in particular by bottom up growth using molecular beam epitaxy and metalorganic vapor phase epitaxy (MOVPE) on patterned substrates, is under investigation – requesting methods for characterizing local properties of the crystal material.

We present results obtained with an FE-SEM which is equipped with secondary electron (SE), In-Beam SE, low-kV backscattered electron (BSE), electron beam induced current (EBIC) and monochromatic CL detection as well as a piezo controlled manipulator setup, c.f. Figure 1. Simultaneous usage of all available detectors and the manipulator is possible, only the BSE and optical detection are physically hindered by another. A modified parabolic collection mirror enables measuring luminescence from planar samples in a tilted view up to 30°, with respect to the large chamber this enables also a nondestructive investigation of full 4’’-wafers.

The electron optics (EO) of this FE-SEM is also capable of rocking the electron beam on a small area, e.g. rocking in a cone of ±12° on an area of about 15 µm in diameter. We will present how the rocking alignment can be adjusted and evaluated by using samples with dedicated contrast structures.

The 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. This enables a fine adjustment of the diffraction conditions and subsequent ECCI images for evaluating the type of defects. 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.

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, see Figure 2. Although having a certain topography contrast also 3D-structures with dimensions of a few µm can be analyzed regarding their crystal structure and orientation using ECP. As CL is quite sensitive also to intrinsic and extrinsic point defects, such correlative images obtained on sidewall and cleavage edges of fin-GaN structures will give valuable insights for discussion of defect mechanism and optical properties.

As the resolution of channeling contrast (e.g. ECCI) and scattering volume (e.g. CL) versus the beam energy are different to another, a correlation of images obtained on the same sample area using different energies improves the identification of individual features. This demonstrates further options for investigating the material quality of 3D structures.

1. Naresh-Kumar, G. et al. Electron channelling contrast imaging for III-nitride thin film structures. Mater. Sci. Semicond. Process. 47, 44–50 (2016). doi: 10.1016/j.mssp.2016.02.007

2. Schimpke, T. et al. Phosphor-converted white light from blue-emitting InGaN microrod LEDs. Phys. status solidi 8, n/a–n/a (2016). doi: 10.1002/pssa.201532904

3. Mandl, M. et al. Group III nitride core-shell nano- and microrods for optoelectronic applications. Phys. status solidi – Rapid Res. Lett. 7, 800–814 (2013). doi: 10.1002/pssr.201307250a

4. Hartmann, J. et al. High Aspect Ratio GaN Fin Microstructures with Nonpolar Sidewalls by Continuous Mode Metalorganic Vapor Phase Epitaxy. Cryst. Growth Des. 16, 1458–1462 (2016). doi: 10.1021/acs.cgd.5b01598

Figures:

Figure 1: Colorized photo of the FE-SEM setup highlighting the sample stage (green), manipulator (cyan), parabolic mirror for light collection (blue), and electron detectors.

Figure 2: SE, BSE and panchromatic CL image at 20 keV (top, aligned for ECCI conditions using a tilt of 15°) and 5 keV (bottom) from the same area of a GaN layer grown by MOVPE on a sapphire substrate.

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EMC Abstracts - https://emc-proceedings.com/abstract/correlative-ecci-and-cl-of-single-gan-microstructures-obtained-using-ecp-by-beam-rocking-on-small-areas/