Summary of [S]/([S] + [Se]) ratios extracted from XPS and Raman-SACS measurements. The techniques have a characteristic 1/e attenuation length λ of ∼1–3 nm (XPS) and 50–60 nm (Raman). While “Raman-front” was measured from the top surface of the samples, “Raman-back” was measured close to the interface with the Mo back contact.

Sample	XPS	“Raman-front” (±0.06)	“Raman-back” (±0.06)
One spike	0.54 ± 0.04	0.01	0.01
Two spikes	No data	0.19	0.15
Three spikes	0.66 ± 0.05	0.25	No data