Electron Channeling Contrast Imaging of crystal phase engineered III-V nanowires

Session

PSA.3 - Semiconductors & Devices

Authors

Dr. Sebastian Lehmann (1), Dr. Daniel Madsen (1), Dr. Stefan Zaefferer (2), Prof. Kimberly Dick Thelander (1)

Affiliations

1. Lund University
2. Max-Planck-Institut fuer Eisenforschung

Keywords

ECCI, III-V nanowires, wurtzite, zinc blende

Abstract text

Summary

In this work we show that electron channeling contrast imaging (ECCI) in the SEM is a powerful method to image segments of different crystal structure in III-V nanowires. In combination with Kikuchi pattern simulations we investigated the contrast formation mechanism and studied the resolution limit of ECCI in the SEM by correlating it to transmission electron microscopy (TEM) measurements of the identical specimen. Although the main emphasis will be on the analysis of InAs nanowires, we also present data on GaAs and InP nanowires.

Introduction

When engineering the crystal phase along the growth axis of III-V nanowires crystal phase quantum dots can be fabricated[1-3]. However, even if precise crystal phase tuning is available, the real structure of electrically or optically characterized nanowire-based devices has to be probed after the corresponding characterization to verify the results. In contrast to e.g. photoluminescence studies where nanowires can be placed on TEM compatible grids, this is usually not the case for standard configurations of e.g. electrical characterization. Therefore, an alternative strategy for the crystal phase analysis is needed. The ECCI method is established predominantly in the field of crystal defect imaging and has shown its great potential there[4]. However, whether or not it can be applied to crystal phase tuned nanowires and where the resolutions limits lie has so far not been investigated. 

Methods/Materials

InAs, GaAs, and InP nanowires investigated in this study were prepared by metal organic vapour phase epitaxy in an AIXTRON 3x2” close coupled showerhead (CCS) system (InAs, GaAs) and in an AIXTRON 200/4 horizontal system (InP) following the vapour-liquid-solid growth approach[5]. The crystal phase of the III-V materials can be precisely controlled along the 〈111〉_B-type growth axis of the nanowire being either zincblende or wurtzite by changing the group V precursor flow and thus the nominal V/III-ratio in the growth chamber[6-8]. 

ECCI data was acquired in a ZEISS Merlin station operated at 30 kV and a probe current of 2 nA with a Backscatter electron detector (BSD) and an Everhart-Thornley secondary electron (SE) detector while for standard SEM imaging a ZEISS LEO Gemini 1560 setup operated at acceleration voltages of 15-20 kV using an InLens SE detector.

A JEOL 3000F transmission electron microscope (TEM) was used for the structural characterization of the nanowires. Simultaneous acquisition of SE images and the orientation correlation by Kikuchi patterns was carried out in a Hitachi HF 3300S TEM operated at 300 kV and equipped with an additional SE detector. For Kikuchi pattern simulation we used the TOCA simulation software tool[9]. 

Results and Discussion

The crystal structure of InAs nanowires was engineered along the <111>_B-type growth axis varying between wurtzite and zinc blende, the latter with its two rotational twin options. Of two of these nanowires, after being placed onto standard, lacey carbon covered Cu-grids, BSE images were acquired with a BSD detector for tilt series of the respective samples. After that acquisition, the identical nanowires were analyzed by high resolution TEM for correlating the structural properties of the nanowires with the ECC images and to further probe the spatial resolution limit of ECCI. We further acquired, again of the identical sample(s), Kikuchi patterns and SE-images in a TEM to probe the crystal structure orientation with respect to the electron beam. Together with TOCA simulations we could correlate the crystal structure and its respective orientation to the electron beam with differences of the ECC formation of the respective crystal phases. Finally, we extended the study to crystal phase engineered GaAs and InP nanowires to show the suitability of ECCI to other material systems as well.  

Conclusion

Distinguishing between different crystal structures in III-V nanowires was successfully shown by using the ECCI technique in a SEM. Nanowire segments of wurtzite and the two rotational twin orientations of zinc blende could be distinguished by orienting the nanowires specifically to the incoming electron beam of the SEM. The crystal structure orientation with respect to the incident electron beam was verified by means of Kikuchi pattern imaging in a TEM and simulations by the TOCA software package[9]. The resolution limit of crystal phase distinguishing using ECCI was determined to be better than 5 nm based on ECCI/SEM and TEM correlation. Overall the ECCI technique was shown to have a great potential for imaging nanowire samples but is of course not limited to this type of samples. The technique is not limited to nanowire architectures and could be very suitable for any type of sample/device which exhibits crystal structure changes and which is embedded in a non-electron-transparent architecture such that no TEM study is possible.

References

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