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Overcoming Low Ge Ionization and Erosion Rate Variation for Quantitative Ultralow Energy Secondary Ion Mass Spectrometry Depth Profiles of Si1–xGex/Ge Quantum Well Structures

Morris, Richard J. H., Dowsett, Mark G., Beanland, Richard, Dobbie, Andrew, Myronov, Maksym, Leadley, David R.
Analytical chemistry 2012 v.84 no.5 pp. 2292-2298
chemical elements, ionization, ions, mass spectrometry, transmission electron microscopy
We specify the O₂⁺ probe conditions and subsequent data analysis required to obtain high depth resolution secondary ion mass spectrometry profiles from multiple Ge/Si₁–ₓGeₓ quantum well structures (0.6 ≤ x ≤ 1). Using an O₂⁺ beam at normal incidence and with energies >500 eV, we show that the measured Ge signal is not monotonic with concentration, the net result being an unrepresentative and unquantifiable depth profile. This behavior is attributed to a reduced Ge ionization rate as x approaches 1. At lower beam energies the signal behaves monotonically with Ge fraction, indicating that the Ge atoms are now ionizing more readily for the whole range of x, enabling quantitative profiles to be obtained. To establish the depth scale a point-by-point approach based on previously determined erosion rates as a function of x is shown to produce quantum well thicknesses in excellent agreement with those obtained using transmission electron microscopy. The findings presented here demonstrate that to obtain reliable quantitative depth profiles from Ge containing samples requires O₂⁺ ions below 500 eV and correct account to be taken of the erosion rate variation that exists between layers of different matrix composition.