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Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate

View Article: PubMed Central - PubMed

ABSTRACT

The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelectric lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM – electrons reflected) to Low Energy Electron Microscopy (LEEM – electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides experimental evidence for a local stray, lateral electric field.

No MeSH data available.


(a) PFM-topography image of the sample. (b) PFM-phase image of the same area. (c) Height profile along the red line on the topography image, and the corresponding local slope.
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f1: (a) PFM-topography image of the sample. (b) PFM-phase image of the same area. (c) Height profile along the red line on the topography image, and the corresponding local slope.

Mentions: The experiments were performed using a LEEM III microscope (Elmitec). The incident electron beam was emitted by a thermionic LaB6 electron gun at an accelerating voltage of U0 = −20 kV. The sample was at U0 + SV, where the Start Voltage (SV) defines the incident electron energy with respect to the sample surface. At low start voltages (SV → 0 V), the incident electrons are reflected by the potential above the surface, i.e. Mirror Electron Microscopy (MEM). At higher voltages (typically, 1 V < SV < 10 V) they penetrate the sample and backscattering occurs, i.e. Low Energy Electron Microscopy (LEEM). The position of the MEM-LEEM transition measures the surface potential. The reflected or backscattered electrons are reaccelerated into the objective lens and finally pass through the imaging column. A double multi-channel plate, screen and camera recorded the electron intensity as a function of SV in a typical field of view of a few tens of microns. An angle-limiting aperture in the back focal plane of the objective lens cut off highly deviated electrons and improves spatial resolution. A schematic of the setup is shown in Fig. 1 of ref. 23.


Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate
(a) PFM-topography image of the sample. (b) PFM-phase image of the same area. (c) Height profile along the red line on the topography image, and the corresponding local slope.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5016809&req=5

f1: (a) PFM-topography image of the sample. (b) PFM-phase image of the same area. (c) Height profile along the red line on the topography image, and the corresponding local slope.
Mentions: The experiments were performed using a LEEM III microscope (Elmitec). The incident electron beam was emitted by a thermionic LaB6 electron gun at an accelerating voltage of U0 = −20 kV. The sample was at U0 + SV, where the Start Voltage (SV) defines the incident electron energy with respect to the sample surface. At low start voltages (SV → 0 V), the incident electrons are reflected by the potential above the surface, i.e. Mirror Electron Microscopy (MEM). At higher voltages (typically, 1 V < SV < 10 V) they penetrate the sample and backscattering occurs, i.e. Low Energy Electron Microscopy (LEEM). The position of the MEM-LEEM transition measures the surface potential. The reflected or backscattered electrons are reaccelerated into the objective lens and finally pass through the imaging column. A double multi-channel plate, screen and camera recorded the electron intensity as a function of SV in a typical field of view of a few tens of microns. An angle-limiting aperture in the back focal plane of the objective lens cut off highly deviated electrons and improves spatial resolution. A schematic of the setup is shown in Fig. 1 of ref. 23.

View Article: PubMed Central - PubMed

ABSTRACT

The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelectric lithium niobate, which presents a large 3.8&thinsp;eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM &ndash; electrons reflected) to Low Energy Electron Microscopy (LEEM &ndash; electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides experimental evidence for a local stray, lateral electric field.

No MeSH data available.