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Elemental And Electronic Characterization Of Semiconductor Materials With The STEM

The Scanning Transmission Electron Microscope routinely forms a 0.5nm diameter probe of 100KeV electrons with a total current of about 1 nano-ampere. It therefore promises to extend to atomic interfaces techniques that have been routinely used in the SEM for micron sized interfaces. An example of As segregation to grain boundaries in poly-crystalline Si is presented to show the strengths and weakness of the present STEM EDS systems. Electron energy loss scattering under the rather special conditions encountered in the STEM is reviewed. This technique promises a spatial resolution that is comparable to the probe size. But the interpretation of the inelastic scattering is complicated by the complex spatial frequencies which are present in the probe, and by the coupling of these frequencies to those in the probed system. The spatial quantization of the bulk plasmon within a small metal sphere is observable using this effect. When the probe passes close to, but not through, a small object, finite coupling can occur to produce energy loss. Surface plasmons with strong dipole symmetry have been observed at optical frequencies in 20nm sized systems with the STEM. Finally, these techniques should be extendible to electronic properties with the advent of new higher energy resolution electron spectrometers. A Wien filter capable of 0.2eV resolution and an accuracy of 20meV over a 1KeV range is described briefly.