[CCoE Notice] Seminar: Characterizations of semiconductor nanomaterials via transmission electron microscopy technology * MH 180, Bauer Business School * 10:30 am, Friday, September 27, 2019 * Guanhui Gao * Rice University *

[Knudsen, Rachel W]

***** Seminar *****
Department of Electrical and Computer Engineering
Materials Engineering Program
Center for Integrated Bio and Nano Systems
  September 27, 2019
10:30 a.m., Room: MH 180
Characterizations of semiconductor nanomaterials via transmission electron microscopy technology
Guanhui Gao
Materials Science and NanoEngineering Department
Rice University
Abstract: Undoubtedly, ex-situ and in-situ (scanning) transmission electron microscopy ((S)TEM) is a versatile and powerful tool to characterize nano-materials with atomic resolution, understanding their microstructure including grain boundaries, line or point defects, structural distortions, i. e., the correlation between structure and property. Beyond that, it is possible to observe the structure and interface between as-grown materials and substrates by means of in-situ TEM. For instance, GaN nanowires (NWs) can be synthesized on substrates such as metallic thin films or multilayer graphene. These metallic substrates offer attractive advantages compared to conventional substrate materials, particularly, a high electrical conductivity allowing, for example, the formation of buried contacts. However, the direct growth of inorganic semiconductors on metallic surfaces is challenging dueto interfacial reactions. Here, we use sputtered Ti films on Al2O3(0001) and multilayer graphene on SiC(000-1) as substrates for vertically aligned GaN nanowires prepared by molecular beam epitaxy (MBE). The major goal of the present study is to study the interface structure between the nanowires and the respective substrates and to explore its role for nanowire alignment and strain relaxation processes.
Cross-sectional specimens for transmission electron microscopy (TEM) are prepared in order to examine the microstructure and chemistry of the interface. One of the key challenges of specimen preparation is to avoid contamination or chemical reactions during the samples’ milling, polishing and thinning process, thus maintaining the original as-grown interface structure. High-resolution and analytical TEM, energy dispersive x-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS), were performed in Titan microscopes operating at 300 kV. EDXS maps and EELS scans demonstrate the incorporation of N in the upper region of the Ti film, resulting in the formation of a thin but continuous TiN layer. High-resolution (scanning) dark-field and bright-field TEM images reveal that the hexagonal GaN NWs are epitaxially aligned to the single crystalline cubic TiN layer with an orientation relationship of (111) TiN || (0001) GaN and [110] TiN || [11-20] GaN. The interface is characterized by the presence of nano-twins and edge-type dislocations as a result of the large mismatch in lattice constants between GaN and TiN of about 6.4%.
Despite the expected existence of only weak van der Waals bonds between the GaN NWs and multilayer graphene, an x-ray diffraction analysis revealed the existence of a strict epitaxial relation between the NWs and the substrate. This is possibly a consequence of the damage and etching of multilayer graphene during exposure to the N plasma used for GaN NW growth, which likely introduces dangling bonds at the graphene surface. The interface is characterized by the absence of localized misfit dislocations and the first GaN layer already exhibits its relaxed bulk lattice constant. Our study demonstrates that both TiN films and multilayer graphene are promising substrates for the fabrication of vertically aligned GaN NWs on materials with metallic conductivity.
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Bio:  Dr. Guanhui Gao (Gigi) was born in Inner Mongolia (Mongolian Minority), China in 1982. She was a joint Ph.D. student in Prof. Ajayan’s group from 2009 to 2012. Then worked in the Shenzhen Institute of Advanced Technology at the Chinese Academy of Science as an associate professor from 2012 to 2015. From 2015 to 2019, she worked at the Paul-Drude Institute für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin, Germany, as a junior scientist. In April 2019, she re-joined Rice University in the Department of Materials Science and NanoEngieering. At Rice, she is in charge of the Titan themis3 STEM in the Electron Microscopy Center, while also focusing on her own research in the interdisciplinary field of materials science and electrochemistry. She is particularly interested in the characterization of and the correlation between structures and properties for nanomaterials and heterostructures via TEM technologies to better understand and enable optimization of the synthesis and performance of nanomaterials. More publications, please visit Dr. Gao google scholar link:
https://scholar.google.com/citations?user=hugTjRIAAAAJ&hl=en
Contact Prof. Francisco Robles Hernandez (fcrobles at Central.UH.EDU<mailto:fcrobles at Central.UH.EDU>) if you would like to meet with Dr. Gao.
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