1068 Enrico Di Russo et al.


(MCP/aDLD) with a detection efficiency ηMCP≈0.6 (Da Costa et al., 2005, 2012). APT analyses were performed either at constant detection rate (ϕ≈0.0010 – 0.0035 event/pulse), varying both applied bias VDC and laser energy Elas,orat constant applied bias VDC with the laser energy Elas varying. The specimen base temperature (T) was 50 or 60 K.


Supplementary Material


Supplementary Material can be found online. Please visit journals.cambridge.org/jid_MAM.


RESULTS


Mass Spectra Selected mass spectra from the series of measurements acquired at constant detection rate are reported in Figure 1. The mass spectrum obtained for the measurement carried


out for Elas=3.4nJ with IR light at the wavelength λ=1,030nm is shown in Figure 1b. The twopeaks associatedwithGa+ (Ga2+, respectively) isotopes are observed at 69 and 71Da (35 and 35.5, respectively). A peak associated with As appears at 75Da (37.5, respectively) for the 1+ (2+, respectively) ionization state. Finally, 113 and 150Da were assigned to As3


2+ and As2 +,


respectively, molecular ions. The presence of these As molecular ions suggests that As2


2+ ions could also form, giving rise to an


overlap with the peak at 75 Da. In this work, the peak at 75Da was associated to As+ only, as in Grieb et al. (2014). A similar potential ambiguity affects the peak at 150 Da, which could be associated to both As2


arguments, and further justified in the following, the peak at 150Da was entirely associated to As2


+ and As4


2+. However, based on similar +.Finally, itshould benoted


that when using low Elas and high VDC, the spectrum is affected by a significant background noise, which is related to DC field evaporation. Figure 1c shows the mass spectrum obtained for Elas=


65.3 nJ (IR mode) and T=50 K. Applying a lower voltage VDC (higher Elas), leads to a drastic drop of the background noise. However, the use of higher laser energy to maintain the detection rate leads to important thermal tails in the spectrum, which is attributed to a long cooling time of the tip after laser irradiation. This points to a possible diminution in the efficiency of heat dissipation for increasing incident laser energy. Furthermore, new molecular ions appear, such as As3


and 188 Da, suggesting that also As4 + and As5


ions are formed. In order to check the role of the laser wavelength, we also carried out a measurement using green laser light


+ molecular ions detected at 225Da and two peaks at 300 2+ molecular


Figure 1. GaAs mass spectrum measured at constant detection rate using: (a) Laser pulse energy Elas=0.03 nJ (green mode), effective field Feff≈21V·nm−1, temperature T=60K, detection rate ϕ≈0.0010÷0.0020 event/pulse; (b) Elas=3.4nJ (IR mode), Feff≈23V·nm−1,T=50 K, ϕ≈0.0020÷0.0035 event/pulse; (c) Elas=65.3nJ (IR mode), Feff≈23V·nm−1, T=50 K, ϕ≈0.0020÷0.0035 event/pulse. The number of ions detected for each dataset is 2.75 ·105.


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