1070 Enrico Di Russo et al.
series are displayed in Figure 3b. For a variation of the laser energy comparable to that explored in the constant detection rate series, XGa exhibits a significantly weaker evolution around the value XGa=0.46±0.03, which indicates the main physical parameter determining the compositional bias is the electric field, rather than the laser energy. In the constant detection rate experiment, Figure 3a, the
results show that the number of molecular ions increases when the electric field decreases. In the series of measure- ments performed at constant VDC=5 kV, the As-CSR was kept nearly constant at 0.62±0.12. The evolution of the abundances of the different As molecular ions, depicted in Figure 3b, clearly shows that laser energy does not affect significantly the As clustering. Large As-molecular ions are not detected at high field, probably because As-clusters can be formed only at low field. High fields lead to the field dissociation of As-clusters formed on tip surface. In the experimental conditions adopted in this work we
early study, these were later confirmed by Gorman et al. (2011). In this article, group V clustering and thermal tails were observed at low fields (high Elas). Moreover, a small amount of As6
have never observed Ga-containing molecular ions. Similar results were reported by Cerezo et al. (1986) in an
+ molecular ions were detected in experiments reported by
Cerezo. This is surprising because in a GaAs ordered lattice As atoms have only Ga atoms as first neighbors and no As–As or Ga–Ga bonds are present. However, while this condition strictly holds in the bulk, it may relax on the surface, where it is known that reconstruction can occur (Biegelsen et al., 1990). Gorman proposed that significant surface atom migration for the group V atom could occur due to laser heating effects (Gorman et al., 2011). This hypothesis is supported by studies of growth by molecular beam epitaxy,where it is reported that groupVatoms have high diffusion coefficients and low activation energies (Neave et al., 1985; Gorman et al., 2011). We notice that the cluster fraction is rather insensitive to the laser pulse energy, which could be explained by the fact that the fraction of eva- porated clusters not only increases with tip temperature and surface diffusion, but should also be related to other surface mechanisms, such as, for example, local field gradients. It is also interesting to compare our results on GaAs
with those published by Müller et al. (2011) for GaSb. In fact, analogies between GaAs and GaSb atom probe measure- ments can be made due to the chemical similarity between As and Sb anions. In Muller’s work, experiments were car- ried out using a green laser (λ=532 nm), with Elas ranging between 0.01 and 0.1 nJ, at T≈43 K. Feff was calculated considering Ga-CSR. Measurements were performed between ~16 and 20 V/nm, which are values lower than those for GaAs (Figs. 3, 4). GaSb has, therefore, a smaller eva- poration field compared to GaAs. Contrary to the case of As, Sb has two isotopes that in mass spectra allow Sbn
distinguished from Sb2n 2n+ molecular ions. Nevertheless, Sb2
n+ to be 2+
molecular ions were never detected. This result suggests that by analogy to GaSb, the peak at 75Da in GaAs can be entirely associated with As+ molecular ions and not to As2
2+ . Similar considerations can be done for the peak at 150 Da. Muller’s
Figure 4. a: Locally-resolved composition measured in GaAs for the series of constant detection rate measurements (ϕ≈0.0010–0.0035 event/pulse), plotted as a function of the local ratio As2+/As+. b: Spatial distribution of the As-CSR and (c) of the Ga fraction acquired at Elas=32.1nJ with the IR laser.
experiments also show that Sb3
formed at relatively low field (high Elas), in comparison with the field dependence formation of As3
2+ and Sb2 + molecular ions are
2+ and As2 + molecular
ions in the case of GaAs. A further insight into the field dependence of the mea-
sured composition is given by the analysis of the composi- tion distribution on the detection area. This can be done by subdividing the detector surface into an array of square regions and measuring the composition in each of them, as described in Mancini et al. (2014). In our case, we used this method by dividing the data-containing region of the detector into 40 square regions. The results are reported in Figure 4 as a function of the
As-CSR. Ga-poor (As-poor, respectively) compositions were measured in regions where the electric field is high (low, respectively). Stoichiometric composition (XGa=0.5) was found only for intermediate fields close to ~22.8 V/nm. Although measurements were performed on different tips, but with similar shapes and dimensions, and using different laser wavelengths, the experimental data lie on a clearly defined curve. This underlines that the As-CSR is a reliable parameter for comparing measurements carried out under different experimental conditions. In contrast, laser pulse energy Elas cannot be directly linked to the energy adsorbed by the tip, as this depends also on the laser wavelength, the tip geometry and the laser spot size (Vella, 2013). The spatial distribution of the As-CSR and of the Ga fraction acquired at Elas=32.1 nJ with the IR laser are
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