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The depth resolution attainable in secondary ion mass spectroscopy (SIMS) depth profiling is shown to be greatly enhanced by the use of a floating ion gun, which is capable of delivering high primary ion currents (ca. 100 nA) at impact energies down to 250 eV. With specially grown, genuinely abrupt GaAs (Si,Al) delta layers and Si0.8Ge0.2/Si structures we have achieved SIMS profile full width at half maximum data as low as 1.3–2.2 nm (species dependent). Approximating the leading edges by exp(z/λL), where z is depth, we achieve λL (growth length) values ≊0.2 nm (i.e., 0.5 nm per decade of concentration). The trailing edge data, exp(−z/λT), exhibit the expected elemental dependence due to differences in preferential sputtering and more complex effects, with λT (decay length) ranging from 0.6 nm for Si to 1.8 nm for Al. This analytical performance, which can be obtained in combination with at least 1 part per million atomic elemental sensitivity, represents a major improvement in SIMS depth profiling capabi...