Zhaofeng Gan

Determination of Polarization‐Fields Across Polytype Interfaces in InAs Nanopillars

Polarization fields within InAs nanopillars with zincblende(ZB)/wurtzite(WZ) polytype stacking are quantified. The displacement of charged ions inside individual tetrahedra of WZ regions is measured at the atomic scale. The variations of spontaneous polarization along the interface normal are related to strain at interfaces of different polytypes. Thus, direct correlation between local atomic structure and electric properties is demonstrated.

Direct mapping of charge distribution during lithiation of Ge nanowires using off-axis electron holography

The successful operation of rechargeable batteries relies on reliable insertion/extraction of ions into/from the electrodes. The battery performance and the response of the electrodes to such ion insertion and extraction are directly related to the spatial distribution of the charge and its dynamic evolution. However, it remains unclear how charge is distributed in the electrodes during normal battery operation. In this work, we have used off-axis electron holography to measure charge distribution during lithium ion insertion into a Ge nanowire (NW) under dynamic operating conditions. We discovered that the surface region of the Ge core is negatively charged during the core-shell lithiation of the Ge NW, which is counterbalanced by positive charge on the inner surface of the lithiated LixGe shell. The remainder of the lithiated LixGe shell is free from net charge, consistent with its metallic characteristics. The present work provides a vivid picture of charge distribution and dynamic evolution during Ge NW lithiation and should form the basis for tackling the response of these and related materials under real electrochemical conditions.

Mapping electrostatic profiles across axial p-n junctions in Si nanowires using off-axis electron holography

Si nanowires (NWs) with axial p-n junctions were grown by the vapor-liquid-solid method. Transmission electron microscopy and electron holography were used to characterize the microstructure and electrostatic properties. Measurement of the potential profile showed the presence of a p-n junction with a height of 1.0 ± 0.3 V. A Schottky barrier was observed at the end of the NW due to the Au catalyst particle. Comparison with simulations indicated dopant concentrations of 1019 cm−3 for donors and 1017 cm−3 for acceptors. These results confirm the benefit of combining off-axis electron holography with simulations for determining localized information about the electrically active dopant distributions in nanowire structures.

Atomic configurations at InAs partial dislocation cores associated with Z-shape faulted dipoles

The atomic arrangements of two types of InAs dislocation cores associated by a Z-shape faulted dipole are observed directly by aberration-corrected high-angle annular-dark-field imaging. Single unpaired columns of different atoms in a matrix of dumbbells are clearly resolved, with observable variations of bonding lengths due to excess Coulomb force from bare ions at the dislocation core. The corresponding geometric phase analysis provides confirmation that the dislocation cores serve as origins of strain field inversion while stacking faults maintain the existing strain status.

Characterization of electrical properties in axial Si-Ge nanowire heterojunctions using off-axis electron holography and atom-probe tomography

Nanowires (NWs) consisting of P-doped Si/B-doped Ge axial heterojunctions were grown via vapor-liquid-solid synthesis using a combination of Au and AuGa catalyst particles. Off-axis electron holography (EH) was used to measure the electrostatic potential profile across the junction resulting from electrically active dopants, and atom-probe tomography (APT) was used to map total dopant concentration profiles. A comparison of the electrostatic potential profile measured from EH with simulations that were based on the APT results indicates that Ga atoms unintentionally introduced during AuGa catalyst growth were mostly electronically inactive. This finding was also corroborated by in situ electron-holography biasing experiments. Electronic band structure simulations guided by the experimental results helped to provide a much better explanation of the NW electrical behavior. Overall, this work demonstrates that the combination of EH, APT, in situ biasing, and simulations allows a more complete understanding o...

Determination of Mean Inner Potential and Inelastic Mean Free Path of ZnTe Using Off-Axis Electron Holography and Dynamical Effects Affecting Phase Determination

The mean inner potential (MIP) and inelastic mean free path (IMFP) of undoped ZnTe are determined using a combination of off-axis electron holography and convergent beam electron diffraction. The ZnTe MIP is measured to be 13.7±0.6 V, agreeing with previously reported simulations, and the IMFP at 200 keV is determined to be 46±2 nm for a collection angle of 0.75 mrad. Dynamical effects affecting holographic phase imaging as a function of incident beam direction for several common semiconductors are systematically studied and compared using Bloch wave simulations. These simulation results emphasize the need for careful choice of specimen orientation when carrying out quantitative electron holography studies in order to avoid erroneous phase measurements.

Measurement of mean inner potential and inelastic mean free path of ZnO nanowires and nanosheet

ZnO nanowires (NWs) and ZnO nano-sheets were grown using the chemical vapor deposition method. The NW structure was characterized using transmission electron microscopy, while the mean inner potential and inelastic mean free path for 200 keV electrons were measured using off-axis electron holography to be 15.3 ± 0.2 V and 55 ± 3 nm, respectively. These values were then used to characterize the thickness of a ZnO nano-sheet, and gave consistent results. This study demonstrates that electron holography can provide useful information about nanostructured ZnO materials and devices.

Characterization of Semiconductor Materials Using Electron Holography

Off-axis electron holography in the transmission electron microscope provides a unique and powerful approach for visualizing electric and magnetic fields within materials with resolutions approaching the nanometer scale. This ability to image phase shifts at medium resolution, and to quantify the corresponding electrostatic profiles, has opened up a wide range of interesting and important semiconductor materials problems for study, particularly in the area of electronic devices where other characterization techniques can no longer be used because of limited resolution [1]. This experimental capability has proven to be invaluable when developing strategies for doping semiconductor devices or when carrying out device modeling and simulations. However, minimizing unwanted artifacts resulting from sample preparation, such as implanted ions or amorphization of surface layers during focused-ionbeam milling, remains an ongoing challenge that must always be addressed when using the technique.

Characterization of Trapped Charge in Ge/LixGe Core/Shell Structure during Lithiation using Off-axis Electron Holography

1. Department of Physics, Arizona State University, Tempe, AZ 85287. 2. Environmental Molecular Sciences Lab, Pacific Northwest National Laboratory, Richland, WA 99352. 3. Device Research Lab., Dept. Electrical Engineering, University California, Los Angeles, CA 90095. 4. Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei City, Taiwan, 10607, Republic of China.

In Situ Biasing of Tapered Si-Ge NW Heterojunctions using Off-Axis Electron Holography

Heterojunction Si-Ge nanowires (NWs) have potential applications such as field effect transistors [1]. Knowledge of the active dopant concentration and distribution, and resultant built-in potential across the Si-Ge p-n junction under biasing conditions is important for improving device performance. Off-axis electron holography is an effective method to measure electrostatic potential with nanoscale resolution [2,3]. By reconstructing phase images from interference holograms, the projected potential distribution of the sample along the electron beam direction can be quantitatively mapped. Here, we have used off-axis electron holography to measure the potential profile across p-n heterojunctions in Si-Ge NWs in situ under varying biasing conditions. The heterojunction Si-Ge NWs are grown using the vapor-liquid-solid method with AuGa catalyst [4], and consist of a B-doped (~10 18 cm -3 ) tapered Ge base and an untapered P-doped (~10 19 cm -3 ) Si tip.