Study of Semiconductor Valence Plasmon Lineshapes via Electron Energy-Loss Spectroscopy in the Transmission Electron Microscope.
Abstract
Electron energy-loss spectra of the semiconductors Si, AlAs, GaAs, InAs, InP, and Ge are examined in detail in the regime of outer-shell and plasmon energy losses (0-100eV). Particular emphasis is placed on modeling and analyzing the shapes of the bulk valence plasmon lines. A line shape model based on early work by Frohlich is derived and compared to single-scattering probability distributions extracted from the measured spectra. Model and data are found to be in excellent agreement, thus pointing the way to systematic characterization of the plasmon component of EELS spectra. The model is applied to three separate investigations. First, variations in the Si plasmon line shape are detected and quantified via least-squares analysis of measured plasmon lines based on the derived model. Broadening of the plasmon line with decreasing specimen thickness is found. The broadening becomes particularly pronounced at thicknesses less than 30nm. Significant broadening is also found in amorphous versus crystalline silicon. No change in the plasmon energy or in the functional form of the plasmon line is observed in association with the line broadening. Next, the line shape analysis technique is employed in the detailed measurement and comparison of the plasmon line shapes of the semiconductors listed above. By comparing the measured plasmon energy with the free-electron plasmon energy of the valence electrons in each case, it is found that, in all but the In semiconductors, the upward shift of the plasmon energy due to coupling with lower-energy interband transitions is dominant over a downward shift due to the polarizability of the ion cores near the plasma frequency. These two effects seem to balance out in the In semiconductors due to the presence of the relatively weakly bound (19eV) In 4f electrons, which enhance the core polarizability at the plasma frequency (14.8eV/ hbar). Finally, since the power-law background model (used to extract ionization edge intensities for EELS elemental analysis) breaks down in the low-loss (20-50eV) regime, the suitability of the plasmon model as a replacement for the power law in this particular case is examined. Indications are that low-energy ionization edges, such as GaM _{4,5} or In N_ {4,5} can indeed be extracted from the strong plasmon background upon which they ride.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1988
- Bibcode:
- 1988PhDT.......124K
- Keywords:
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- Physics: Condensed Matter