“…In the samples with higher deposition times, that signal was shifted towards lower BE, to the typical value of fluorides at BE = 684.0 ÷ 685.0 eV. It is well known 15 that the surface cleaning of GaAs substrate with HF leads to the formation of GaF, GaF 3 and As 2 O 3 in the outmost layer of GaAs, but our results excluded the presence of Ga fluorides on GaAs reference sample before BaF 2 deposition (Table 1). Furthermore, volatile compounds of Ga fluorides can be formed as a consequence of the reaction between BaF 2 vapour and GaAs substrate, as described in Stumborg et al 16 This effect was observed in the initial stage of BaF 2 deposition (samples BF2 and BF3), where a low intensity peak of GaF 3 at BE = 684.9 eV was observed 17 .…”