In-situ Transmission Electron Microscopy of Crystal Growth under MOVPE Conditions
The presented work introduces in-situ (scanning) transmission electron microscopy ((S)TEM) using a closed gas cell and heating TEM holder as a new experimental approach to investigate crystal growth on an atomic scale under metal-organic vapor phase epitaxy (MOVPE) comparable conditions. This bridge...
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|Summary:||The presented work introduces in-situ (scanning) transmission electron microscopy ((S)TEM) using a closed gas cell and heating TEM holder as a new experimental approach to investigate crystal growth on an atomic scale under metal-organic vapor phase epitaxy (MOVPE) comparable conditions. This bridges the gap from in-situ investigations performed in environmental TEMs (ETEMs) comparable to molecular beam epitaxy (MBE) towards the industrially relevant MOVPE processes. The setup enables the safe handling of toxic and pyrophoric gases, such as metal-organic precursors utilized in the growth of semiconductor materials. A proof of concept is given by thermal decomposition studies of the precursor molecules tertiary butyl phosphine (TBP) and trimethyl gallium (TMGa) and comparing these findings to those obtained in industrial reactor designs. Moreover, live observations of the growth of GaP nanowires in zinc blende structure by gold-catalyzed vapor-liquid-solid (VLS) growth using TBP and TMGa were performed, demonstrating the versatile capabilities of the technique and setup. Thereby, its dynamic was investigated, revealing unique insights into the growth process. Thermal precursor decomposition was analyzed by mass spectrometry utilizing an inline quadrupole mass spectrometer. The unimolecular and bimolecular reaction pathways and the respective decomposition temperatures of TBP and TMGa were revealed through methodical data analysis. These conclusions are compatible with results obtained in conventional reactor designs, and the setup has been shown to be an appropriate model for an MOVPE reactor on a micrometer scale. In-situ STEM observations of gold-catalyzed VLS growth of GaP nanowires using TBP and TMGa were performed under MOVPE-comparable conditions. In order to handle vast amounts of image data gathered during the STEM video recordings in a reasonable time, an automated MATLAB-based data evaluation was created. Important geometric properties of the nanowires, such as diameter, areas of the phase boundaries, as well as growth rate, are determined and synchronized with the experimental parameters, such as growth and imaging conditions. It was shown that the utilized growth conditions lead to diameter-independent growth kinetics. The precursor partial pressure and the ratio of the vapor-liquid and liquid-solid interfaces solely determined growth rates. A transition from group V to group III limited growth occurs that aligns with the growth kinetics observed in MOVPE. The confirmation of this transition at V/III ratios also observed in MOVPE, represents an essential difference to other in-situ studies performed in ETEMs under high vacuum conditions more comparable to MBE and highlights the importance of a setup capable of operating at MOVPE conditions. The dependence of the growth rate on the surface ratio has not been reported in the literature so far. Revealing this dependency was enabled through live monitoring of the growth process and the availability of many different wire geometries simultaneously under the same conditions. In dealing with nanowire growth rates, the introduced normalized growth rate as a fraction of growth velocity and surface ratio should be considered in the diffusion-limited regime. In addition, nanowire kink formation could be observed as it was happening. In agreement with the in-situ observations, post-growth investigations by scanning precession electron diffraction measurements confirmed that the formation of kinks could be related to twin formation. A substrate preparation method was developed to improve further the in-situ VLS growth observation that facilitates epitaxial growth in zone axis imaging conditions. The method is based on mechanical abrading the substrate, conserving a nanometer-thin gold layer deposited on its crystalline surface, and FIB transfer onto a microelectromechanical system chip. With the advantages of this technique, investigations with a predetermined zone axis orientation of growing nanowires become possible, opening the door to live imaging of crystal growth at atomic resolution. While the experimental proof is still pending, the advantages of this technique promise more profound insights into growth kinetics by examining, for example, the nucleation of crystalline layers or the formation of crystal defects. Thereby lays the foundation for crystal growth observations under MOVPE-comparable conditions in a TEM. Besides successfully demonstrating functionality and delivering new insights into the VLS growth process, this method promises a wide variety of possible avenues of investigation. Overall, the groundwork on closed gas cell in-situ TEM laid in this work is expected to lead to many new insights into the VLS growth process and deepen the understanding of the MOVPE process in general.|
|Physical Description:||197 Pages|