Development of nanotopography during SIMS characterization of thin films of Ge_1-xSn_x alloy

This work presents a study of application of secondary ion mass spectrometry (SIMS) to measure tin concentration in Ge_1-xSn_x alloy with x higher than solid solubility ∼1%, i.e. well above the diluted regime where SIMS measurements usually provide the most reliable quantitative results. SIMS analysis was performed on Sn⁺ ion implanted Ge films, epitaxially deposited on Si, and on chemical vapor deposition deposited Ge_0.93Sn_0.07 alloy. Three SIMS conditions were investigated, varying primary beam ion species and secondary ion polarity keeping 1 keV impact energy. Best depth profile accuracy, best agreement with the fluences measured by Rutherford backscattering spectrometry, good detection limit (∼1 × 10¹⁷ at/cm³) and depth resolution (∼2 nm/decade) are achieved in Cs⁺/SnCs⁺ configuration. However, applied sputtering conditions (Cs⁺ 1 keV, 64° incidence vs. normal) induced an early formation of surface topography on the crater bottom resulting in significant variation of sputtering yield. Atomic force microscopy shows a peculiar topography developed on Ge: for oblique incidence, a topography consisting in a sequence of dots and ripples was observed on the crater bottom. This behavior is unusual for grazing incidence and has been observed to increase with the Cs⁺ fluence. Rotating sample during sputtering prevents this ripple formation and consequently improves the depth accuracy.