Field-Dependent Measurement of GaAs Composition 1071
reported in Figures 4b and 4c, respectively, in the detector space. The specimen side illuminated by the laser is imaged on the top side of the detector space. No effect of the laser incidence can be observed on the distribution maps, which evidence rather a nearly radial distribution, with a low-field region close to the specimen axis. The distributions reported in Figures 4b and 4c also visually confirm the correlation between field intensity and Ga fraction reported in Figure 4a. The loss of Ga may therefore be expressed in terms of
preferential evaporation of Ga atoms at high DC electric fields. This is in good agreement with the results reported in
different references for GaN (Agrawal et al., 2011; Diercks, 2013; Mancini et al., 2014) and for AlGaN alloys (Rigutti et al., 2016). The preferential evaporation of Ga in GaAs is consistent with its much lower evaporation field (15 V/nm) compared to that of As (42 V/nm), although this argument is based on values calculated for the different atomic species considered as atom bonded at the surface of pure materials (Tsong, 1978). This is also consistent with the increase of the background noise at high field recorded in the mass spectra of Figure 1, indicating that preferential evaporation sets on. The loss of As atoms at low fields requires more care in
interpretation. The fraction of As originating from the different molecular ions present in themass spectra at constant detection rate and of the constant DC voltage series are reported in Figures 3a and 3b, respectively. A reasonable hypothesis is that thelossofAsatoms at lowfields is either due to the desorption of As neutral clusters fromthe tip surface or to the field dissociation of heavy As molecular ions into neutral and ionic daughter molecules. The fragmentation of these can indeed generate neutral As atoms that are not detected (Gault et al., 2016).
Analysis of Multiple Detection Events
Multiple-ion events correspond to the detection of two or more ions generated on the same laser pulse. The origin of multiple-ion events in APT has been widely discussed by Da Costa et al. (2012) and De Geuser et al. (2007). The analysis of multiple detection events is extremely useful to study the complex processes occurring during field evaporation of compound systems. The fraction ofmultiple events in constant detection ratemeasurement is reported in Figure 5 for various laser energies (i.e., various electric fields). It should be noted that the fraction of multiple events of order n=1, 2, 3,… corresponds to the number of events corresponding to the detection of n ions evaporated on the same laser pulse, and should not be confused with the fraction of ions detected withinmultiple events of order
n.The increase of Feff (decrease of Elas) leads to a progressive rise of multiple events from 9 to 28%. Notice that the slight decrease in the fraction ofmultiple events when changing the laser fromIR to green could also be due to the fact thatmeasurements in greenwere performed at a lower detection rate compared with the IR mode. The histograms of the frequencies of the detection dis-
tances d between multiple-ion hits are given in Figure 6a. A common behavior is observed whatever the laser energy (i.e., whatever the field): the distribution of impact distances
Figure 5. Histogram of the fraction of multiple events for con- stant detection rate measurements performed at different Elas.
Figure 6. a: Histograms of distances between impacts associated with multiple-ion events for different laser pulse energy at con- stant detection rate. b: Histograms of distances between impacts associated with multiple-ion events associated to {69Ga, 69Ga}, {71Ga, 69Ga} and {71Ga, 71Ga} couples.
on the detector (d) is bimodal with two contributions. We observe a peak at small distances close to 2.7mm(correlated events) and a broader distribution centered at larger dis- tances (35mm). Small distances are related to strong spatial correlations in the evaporation process. The analysis of correlated evaporation phenomena
allows the drawing of important conclusions about the biases induced by specific detector limitations, which should also be considered for a correct interpretation of data. The so-called pile-up effect occurs due to the correlated evaporation of neighbor atoms at the tip surface. In brief, the evaporation of a first atom leads to a field enhancement that may provoke the correlated evaporation a neighbor atom. If the second
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