Compositional accuracy of atom probe tomography measurements in GaN: Impact of experimental parameters and multiple evaporation events

Abstract

A systematic study of the biases occurring in the measurement of the composition of GaN by Atom Probe Tomography was carried out, in which the role of surface electric field and laser pulse intensity has been investigated. Our data confirm that the electric field is the main factor influencing the measured composition, which exhibits a deficiency of N at low field and a deficiency of Ga at high field. The deficiency of Ga at high field is interpreted in terms of preferential evaporation of Ga. The detailed analysis of multiple evaporation events reveals that the measured composition is not affected by pile-up phenomena occurring in detection system. The analysis of correlation histograms yields the signature of the production of neutral N due to the dissociation of GaN ions. However, the amount of N neutral molecules that can be detected cannot account for the N deficiency found at low field. Therefore, we propose that further mechanisms of neutral N evaporation could be represented by dissociation reactions such as GaN→ Ga+ N and GaN→ Ga+ N.

Electric field strength-dependent accuracy of TiAlN thin film composition measurements by laser-assisted atom probe tomography

“…An increase from approximately 25-28 V nm −1 improves the accuracy of laser-assisted APT from 11.4 to 4.1 at% for N, from 8.8 to 3.0 at% for Al and from 2.8 to 0.9 at% for Ti. This correlation is in agreement with field evaporation of GaN at low electric field strength [14] since dissociation of molecular ions during the flight leads to the formation of neutral fragments and the electric field is not sufficient to induce ionization thereof [13]. Evidence of dissociation is not observed in the present data since the LEAP 4000X HR is equipped with a reflectron which compensates energy discrepancies of particles with lower or higher energies than expected [23].…”

“…However, accurate quantification represents a major challenge for APT analysis since the obtained absolute concentrations are affected significantly by the measurement parameters. A prominent example is GaN which has been intensely studied by laser-assisted APT and shows significant deviations from the N/Ga=1 stoichiometry [9][10][11][12][13][14]. Despite experimental differences of these investigations with respect to employed instruments and measurement parameters, the reported variations of the N/Ga ratio between 0.05 and 1.63 emphasize the measurement parameter-dependent accuracy or lack thereof.…”

“…Despite experimental differences of these investigations with respect to employed instruments and measurement parameters, the reported variations of the N/Ga ratio between 0.05 and 1.63 emphasize the measurement parameter-dependent accuracy or lack thereof. The electric field strength was identified to govern the absolute concentration values of GaN since Ga-deficiency was obtained at high electric field strength and associated with preferential evaporation [14], while N-deficiency was observed at low electric field strength and could be explained by the formation of neutral fragments upon dissociation of N 2 -carrying molecular ions [13]. The stoichiometric composition N/Ga=1 was obtained for the Ga 2+ /Ga + ratio of approximately 0.04 which corresponds to an electric field strength of approximately 23.5 V nm −1 [14].…”

“…The electric field strength was identified to govern the absolute concentration values of GaN since Ga-deficiency was obtained at high electric field strength and associated with preferential evaporation [14], while N-deficiency was observed at low electric field strength and could be explained by the formation of neutral fragments upon dissociation of N 2 -carrying molecular ions [13]. The stoichiometric composition N/Ga=1 was obtained for the Ga 2+ /Ga + ratio of approximately 0.04 which corresponds to an electric field strength of approximately 23.5 V nm −1 [14]. In case of Al 0.44 Ga 0.56 N the highest N/(Al+Ga) ratio of 0.86 was obtained for the Ga 2+ /Ga + charge state ratio of 0.95 [15] which corresponds to an electric field strength of approximately 24.9 V nm −1 [13].…”

Correlative investigation of Mg doping in GaN layers grown at different temperatures by atom probe tomography and off-axis electron holography

“…The analysis parameters were optimized to operate at low electric field condition on the tip surface and thus to observe the best signal-to-noise ratio in the mass spectrum. 13,14 This reduces also the preferential loss of metallic species during the tip evaporation and allows to better estimate the Mg fraction. 15,16 The 3D reconstruction was performed using a cone-angle algorithm.…”