Field-Dependent Measurement of GaAs Composition 1069


(at a wavelength λ=515 nm) with energy Elas=0.03 nJ, at T=60K and at the detection rate≈0.0010/0.0020 event/pulse. The spectrum obtained is shown in Figure 1a and is quite similar to the one acquired with IR light at low energy. However, the amount of background noise appears to be larger in the mass spectrumdisplayed in Figure 1b. The ability to analyze GaAs both at IR and green wavelengths leads us to suggest that no serious issues should occur with UV illumination, provided a major reduction of the laser intensity can be applied with respect to the typical values used for the analysis of large band semiconductors (e.g., GaN). The mass spectra associated with all constant detection


rate measurements are reported in the Supplementary Material.


Charge-State Ratio and Effective Field Metrics


As demonstrated in previous works, the charge-state metrics are extremely useful for the representation of parameter- dependent composition measurements (Mancini et al., 2014). The ratios of the different charge states of a given species can indeed be used in order to estimate the so-called effective field Feff through the Kingham’s post-ionization model (Kingham, 1982). In principle, this calculation can be done with any species. In the case of GaAs both As charge- state ratio (As-CSR) As2+/As+ and Ga-CSR Ga2+/Ga+ were considered. As shown in Figure 2, the electric fields derived from the Ga- and As-CSRs are close but not equal, suggest- ing that Kingham’s theory cannot be strictly applied to the case under study. Nevertheless, both approaches are straightforward in order to estimate the field, but adopting


Ga-CSR the statistical error becomes high at very low electric field, due to the small amount of measured Ga2+ ions. This is due to the field required to have a Ga-CSR=1, that is more than 25 V/nm, while that for having an As-CSR=1is ~22.5 V/nm. As measurements are typically carried out between 20 and 24 V/nm, the As-CSR turns out to be the


most convenient indicator (Gault et al., 2012). Finally, the consistency within ~5% of the Feff calculated based on the Ga and on the As-CSRs also induces us to exclude that a large number of As2


significantly bias the latter quantity. 2+ ions within the 75Da peak could


Dependence of Measured Composition on Experimental Parameters


The Ga fraction (XGa) measured within the series of mea- surements performed at constant detection rate, with the addition of the measurement acquired using green laser light, is plotted in Figure 3a as a function of the As-CSR. The measured Ga fraction varies from 0.55 at low field to 0.35 at high field. This behavior is qualitatively similar to what has been found in other compound semiconductors such as GaN, AlN, and ZnO (Mancini et al., 2014; Amirifar et al., 2015). As it is not possible to discriminate between the role of surface DC field and laser energy from the simple analysis of constant detection rate data, we also performed a series of measurements at constant applied bias VDC=5 kV and T=50 K, with Elas ranging between 7 and 65.3 nJ (IRmode), with no constraint on the detection rate. The results of this


Figure 2. Relation between As2+/As+ and Ga2+/Ga+ ratio. In figure are also reported the values of Feff calculated according the


Kingham’s post-ionization theory. The dotted line indicates where the electric field calculated based on the AS-CSR equals that calculated based on Ga CSR.


Figure 3. a: Fractions of different ion species as function of the tip surface field in constant detection rate measurement. b: Fractions of different ion species as function of the laser pulse energy in con- stant laser pulse energy measurements. These fractions are defined as the number of atoms associated to each ionic or molecular species, divided by the total number of Ga and As atoms detected.


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