Field-Dependent Measurement of GaAs Composition 1073


Figure 8. Correlation tables associated to the dataset acquired at: (a) Elas=3.4 nJ (Feff=∼23V·nm−1); (b) Elas=65.3 nJ (Feff=∼21V·nm−1). In the tables they are also reported the number of ion pairs associated to each species.


No other dissociation tracks related to processes that occur sufficiently far from the tip surface were observed. In fact, dissociation can also take place very close to the tip surface. In this case, it is impossible to distinguish dissociation pro- cesses from multiple-ion events, due to local variations of the surface electric field upon departure of a ion. This problem was recently investigated by Zanuttini et al. through a theo- retical study of the dissociation of ZnO2+ molecules in dif- ferent channels containing charged and neutral species (Zanuttini et al. 2017). According to this study, the dis- sociation of ZnO2+ molecules is possible and it would explain the oxygen-poor composition found at low Feff (Mancini et al., 2014; Amirifar et al., 2015; Zanuttini et al. 2017). However, this dissociation also would take place so


close to the specimen surface that it is impossible to identi- fied it in the correlation diagram (Zanuttini et al. 2017). Similar considerations can be extended to the case of III–V semiconductors, such as GaN, GaSb, and GaAs (Müller et al., 2011; Mancini et al., 2014; Gault et al., 2016). In particular, for the case of GaN it has already been proved that dis- sociation processes can be associated with the production of neutral N molecules (Gault et al., 2016). A second scenario is that the dissociation of As clusters


at the tip surface leads to the formation and direct desorption of neutral subunits. This can explain the As-poor composi- tion measured at low Feff. In this case, the formation of neutrals cannot be detected. However, if, during molecular fragmentation, more than one charged ions are also gener- ated, a careful statistical analysis of multiple hits can provide some clues about the formation of parent molecules. The correlation between ion-pairs involved in multiple-


ion events was studied by adopting the method of the correlation tables described by Saxey (2011). This method consists of comparing the number pij of each ion-pair ij with the expected number of coincidences eij calculated under the hypothesis of independent co-evaporation, so that eij=ei× ej, where ei is the fraction of the ith species in the multiple-


Figure 9. Evolution of the correlation degree associated to the ion pairs {As2


+,As2+ }.


event mass spectrum. Both pij and eij are n×n matrices. The degree of correlation between measured and expected number of ion pairs is provided by the following matrix:


cij = pij - eij eij


; (9)


where cij represents the degree of correlation (when cij>0) or anti-correlation (when cij<0) between ion-pairs. Due to pile-up effects, species are usually anti-correlated with themselves. In order to correct for this effect, the diagonal values cii are artificially set equal to 0. Moreover, similarly to what is made in χ2 tests, low pij entries (<5) are considered as not statistically significant. In Figure 8, the correlation tables associated with measurements performed at high (Fig. 8a) and low (Fig. 8b) Feff are shown. The number of ion pairs for each ion i is also given. The correlation tables reveal that the ion pairs that


involve As molecular ions are typically anti-correlated. If these ion pairs were generated from the dissociation of As clusters on tip surface, the corresponding Asn


m+ clusters exhibit n, m>3 and the daughters As molecular ions should


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