1074 Enrico Di Russo et al.
be correlated. Because this is not observed, we can deduce that dissociation ofAsn
m+ clusters close to the tip surface is an
unlikely scenario. However, the results suggest a strong correlation between
{As2 +,As2+} ion pair. The evolution of this correlation degree as
afunction of Feff is reported in Figure 9. One possible inter- pretation is that As3 clusters are formed at the tip surface and upon leaving they immediately dissociate in As2
+ and As2+
molecular ions. However, increasing Feff this particular process is reduced, probably because fewer As3 clusters are formed. It is interesting to observe that the same result was found
in the case of GaSb for the {Sb2 +,Sb2+ } ion pair (Müller et al.,
2011). The authors show that a strong correlation exists for both high- and low-field condition. This was explained in terms of formation of metastable Sb3
2+ cluster ions and sub-
sequent dissociation. The correlation tables calculated for each data set acquired are reported in the Supplementary Material.
CONCLUSIONS
In summary, we have measured the compositional biases occurring in the APT analysis of GaAs. The surface electric field was found to be the main physical factor introducing a
bias of the measured composition. An As-poor composition was measured at the low-field condition. When increasing the field, a Ga-poor composition was found. Compositional analysis shows some analogies with the cases of GaN end
GaSb. The preferential evaporation of Ga atoms at the high- field condition can explain the low measured atomic frac- tion. At low field, a possible mechanism accounting for the loss of As atoms is the formation of As clusters on the tip surface and their direct dissociation in smaller subunits with the formation of neutral As molecules/atoms. As molecular ions are more abundant at the low-field condition because high-field conditions prevent enhance field dissociation. The clustering of V-element atoms exhibits only a weak depen- dence on the laser pulse energy at fixed DC voltage. The analysis of multiple events reveals that multiple hits
are strongly correlated in space. Ga homo-isotopic couples are lost due to pile-up phenomena, but the amount of this loss channel, estimated as ~2% of the total amount of detected Ga+ ions, is not sufficient to explain the bias in the compo- sition measurement. Moreover, the analysis of correlation tables reveal that multiple events are field-induced and can be caused by the dissociation of metastable clusters lying on the
tip surface. Results suggest that As3 metastable clusters can field evaporate as As3
As2+} ion pairs, in particular at the low-field condition. No evidence of the formation of Asn
3+ and subsequently dissociate in {As2 +,
m n,m>3metastable clusters
emerged, they are probably not stable enough to form. Finally, considering the lack of specific studies on doped
GaAs and on GaAs-related alloys, APT users should pay par- ticular attention to the correct interpretation of measurements performed on these materials, as the measured doping den- sities and site fractions of the alloy components may exhibit
field-dependent biases, similar to those reported in ZnO and AlGaN (Mancini et al., 2014;Amirifar et al., 2015; Rigutti et al., 2016). The problem of the correct measurement of doping density and alloy composition is technologically relevant and should be addressed in further studies.
ACKNOWLEDGMENT
This work was funded by the French National Research Agency (ANR) in the framework of the projects EMC3 Labex AQURATE and ANR-13-JS10-0001-01 TAPOTER.
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