ELECTRIC-MAGNETIC FIELD MAPPING
4D-Scanning Precession Electron Diffraction (4D-SPED) acquisition method in TEM, is currently used in several applications for materials analysis like Orientation & Phase mapping and Strain analysis . 4DSPED is an extension of 4D-STEM technique and it consists in scanning the electron beam over the structure to observe and record at each position a Precession Electron Diffraction Pattern (PED) pattern.
4D-SPED acquisition together with data processing dedicated software is applied to characterize in detail local electric/magnetic fields and built-in potentials in functional materials.
During scanning over the sample, the transmitted beam is deflected though the local electric field by the Lorentz force.
The module of the local electric field translates into the shift of the intensity distribution in the transmitted central electron beam.
Electric filed is relying on COM (center of mass) displacement of the illumination intensity. Beam Precession of 0.1-0.4 degrees is used to get rid of dynamical effects and obtain ED patterns with less noise leading to enhanced results. Electric field calculations are of high interest to study properties of many materials and devices, e.g. transistors, solar cells or sensors, nanowires, batteries, etc.
Data can be collected by scanning over the area of interest with a pseudo-parallel beam within size of 0.7 – 1.5nm (TEM dependent), using small Condenser aperture (~10μm) and convergent angle that can typically vary from 0.5 mrad to 5mrad (or more) with step size equal or half of the probe size, to achieve highest possible spatial resolution (down to 1nm).
The camera length is properly adjusted to visualize properly the transmitted beam (000) displacement.
Beam convergence angle and camera length are related parameters to fit the beam onto the detector window.
Data can be collected either by standard NanoMEGAS external Stingray CCD, or by triggering/synchronizing with high-end direct detection /CMOS cameras (like Gatan, QD, ASI , Dectris or PnCCD detectors) by NanoMEGAS dedicated scan generator.
Center of Mass (COM) algorithm is used to calculate the mean inner potentials between various sample areas
To calculate electric fields, as first step raw (000) beam COM is calculated after setting up the studied area (typical a few square micron size), then a background correction is applied to overcome the effect of the scanning beam shift that may be present.
The resulted background free COMx and COMy are used to obtain the final COMmagnitude and COMphase, from which the apparent electric field profile is obtained along the different sample areas.
Measurement quality of COM shifts is significantly improved by Precession because of the dynamical effect reduction.
- Sample: GaAs/AlAs (001) interface
- The lattice-matched GaAs/AlAs (001) interface was used as hetero-interface model.
- Marburg University, Prof. Kerstin Volz group*
- JEOL 2200 FS
- NanoMEGAS Digistar PED hardware.
- TEM Alpha 3, Condenser Aperture 20μm, spot size 1
- 2mrad beam convergence, PED frequency: 1KHz
- 1000 fps for the DE (PnCCD) camera,
- Out of crystal zone axis: 7o
- Energy filter ON, Step size: 2nm
- Scanning area 1024 x 1024 nm
- Exp time: 1ms, Scan time: 4min 27sec
*Acknowledgement: NanoMEGAS thanks Dr.Sh. Ahmed, Dr.V. Shejarla, Dr.A. Bayer and Prof.K. Volz form Marburg Un. for sample provided and data collection in their lab, and the critical discussion about electric field calculations. NM thanks Dr. Jean-Luc Rouviere, CEA, Grenoble, for the critical contribution in software development.