Microsc. Microanal. 23, 1067–1075, 2017 doi:10.1017/S1431927617012582


© MICROSCOPY SOCIETY OF AMERICA 2017


Field-Dependent Measurement of GaAs Composition by Atom Probe Tomography


Enrico Di Russo, Ivan Blum, Jonathan Houard, Gérald Da Costa, Didier Blavette, and Lorenzo Rigutti*


UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Matériaux, Normandie Université, 76000 Rouen, France


Abstract: The composition of GaAs measured by laser-assisted atom probe tomography may be inaccurate depending on the experimental conditions. In this work, we assess the role of the DC field and the impinging laser energy on such compositional bias. The DC field is found to have a major influence, while the laser energy has a weaker one within the range of parameters explored. The atomic fraction of Ga may vary from 0.55 at low-field conditions to 0.35 at high field. These results have been interpreted in terms of preferential evaporation of Ga at high field. The deficit of As is most likely explained by the formation of neutral As complexes either by direct ejection from the tip surface or upon the dissociation of large clusters. The study of multiple detection events supports this interpretation.


Key words: atom probe tomography, GaAs, composition analysis, metrology, compositional accuracy


INTRODUCTION Laser-assisted atom probe tomography (La-APT) was recently employed in order to study the composition of several semiconductors, such as oxides (ZnO, MgO), nitrides (GaN, AlN), and some ternary alloys (MgZnO, AlGaN) (Mancini et al., 2014; Rigutti et al., 2016; Di Russo et al., 2017). The studies already conducted reveal that measure- ments are affected by important compositional biases. The role of the analysis parameters during the measurements has been thoroughly analyzed. In particular, the main influence of the DC electric field on measured composition was underlined. The principal aim of this work is study compositional


biases in GaAs atom probe analysis. In particular, we inves- tigated the experimental conditions leading to deviations from the expected Ga/As ratio equal to 1 (stoichiometric composition). Early GaAs La-APT investigations suggest that laser pulse energy Elas has a primary role in determining the measured composition, in particular on As molecular ion formation (Nishikawa et al., 1984; Cerezo et al., 1986; Gorman et al., 2011). However, the studies carried out regarding oxides and nitrides indicate that it is the DC electric field that is responsible of the compositional biases found in atom probe measurements (Mancini et al., 2014). In order to clarify this aspect in the case of GaAs, we adopted a specific protocol of analysis in order to discriminate the role of both DC electric field and Elas on the measured compo- sition of Ga and As. Furthermore, we analyzed in particular the detection of multiple events (i.e., those detected after the same laser pulse) in order to assess possible channels of correlated evaporation and molecular dissociation, and their possible influence on the measured composition.


*Corresponding author. lorenzo.rigutti@univ-rouen.fr Received May 10, 2017; accepted August 27, 2017


Our results show that the electric surface field, rather


than the impinging laser energy, drives a compositional bias in the atomic fraction of Ga measured in GaAs. This can change from 0.55 at low field to 0.35 at high field. While we interpret the loss of Ga at high field as due to preferential evaporation (i.e., evaporation not related with the laser pulse) of this element, the loss of As at low field is compatible with the hypothesis of the formation of neutral As com- plexes, either by direct ejection or upon dissociation of larger molecular ions. In the analysis of GaAs, multiple hits represent 10–20% of the detection events, and their analysis can illustrate the complex field evaporation processes taking place, consistent with our hypotheses.


EXPERIMENTAL


In order to perform La-APT, GaAs specimens were prepared by standard lift out procedure followed by annular milling with 30kV Ga ions and standard cleanup procedure with Ga ions at 2 kV(Padalkar et al., 2014; Blumet al., 2016). In order to reduce ion beam damage, GaAs was capped by 200-nm thick Si and 50-nm thick Ag layers. In this way, needle- shaped atom probe tips with ~ 50-nm radius of curvature were obtained. During this scanning electron microscope/ focused ion beam preparation, the axes of atom probe tips were oriented along the [100] direction. Atom probe analyses were performed using a laser-


assisted wide-angle tomographic atom probe operated with both IR and green femto-second laser pulses (350 fs, wave- lengths λ=1,030 and 515nm) (Gault et al., 2006). The repetition frequency was 100 kHz and laser pulse energy Elas ranged between 0.03 and 63.3 nJ. The corresponding peak intensities during the pulse are reported in the Supplemen- tary Material. The detection system used was a specially designed multi-channel plate/advanced delay line detector

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180