STEM-EELS Introduction

GaN002 0The STEM was conceived and first built by Albert Crewe in the late 1960’s at the University of Chicago. [1] He realized that an electron microscope equipped with a high brightness field emission electron gun would be able to create an atom-sized electron beam. Using beam energies of 100-300 KeV, the bulk structure of nanometer sized specimen areas can be examined with very high magnification. This examination can use several analytical signals: large angle elastic scattering -- Annular Dark Field (ADF) imaging; small angle elastic scattering -- Bright Field (BF) imaging; small angle inelastic scattering -- Electron Energy Loss Spectroscopy (EELS); characteristic x-ray production -- Energy Dispersive X-Ray spectroscopy (EDS); and many others. At IBM, Philip Batson has used a VG Microscopes STEM to explore Spatially Resolved EELS techniques using the very small probe. In order to examine energy scales relevant to semiconductor operation, Batson built a high resolution electron spectrometer in 1986. Achieving 70 meV resolution with an absolute accuracy of 20 meV at 120 KeV beam energy, this instrument has been a leader in EELS spectroscopy of nanoscale areas. Recently, in collaboration with O. Krivanek and N. Dellby of Nion Co., aberration correction optics were successfully incorporated in the IBM STEM, allowing a sub-Angstrom sized electron beam to be created for the first time. [2] Finally, with P. Kruit and W. Mook of the Technical University of Delft, an electron monochromator has been constructed for the VG STEM. With this device the energy line width of the electron beam will be reduced from 300 meV to 60 meV, using electron optics designed to minimize loss of brightness which would compromise the ability to make a sub-Angstrom probe. [3]

Image: Aluminum Nitride: The relative positions of Al columns (red) and N columns (blue) determine the electrical polarization direction.[4]

[1] A.V. Crewe, M. Isaacson, and D. Johnson, A Simple Scanning Electron Microscope, Rev. Sci. Inst. 40, 241-246 (1969).

[2] P.E. Batson, Niklas Dellby, and O.L. Krivanek, Sub-Angstrom resolution using aberration corrected electron optics, Nature 418, 617-620 (2002).

[3] H.W. Mook, P.E. Batson, and P. Kruit, Monochromator for high brightness electron guns, in 12th European congress on electron microscopy, Vol. III, (2000) pp. 315 -316.

[4] K.A. Mkhoyan, P.E. Batson, J. Cha, W.J. Schaff, and J. Silcox, Direct Determination of the Local Polarity in Wurzite Crystals, Science 312, 1354 (2006).