Understanding the Nature of Crystallographic Bonds by Establishing the Correlation between Ion-Pair Chemistry and Their Separation in Detector Space.

The occurrence of multi-hit events and the separation distance between multi-hit ion pairs field evaporated from III-nitride semiconductors can potentially provide insights on neighboring chemistry, crystal structure, and field conditions. In this work, we quantify the range of variation in major III-N and III-III ion-pair separation to establish correlations with bulk composition, growth method, and ion-pair chemistry. The analysis of ion-pair separation along the AlGaN/GaN heterostructure system allows for comparison of Ga-N and Ga-Ga ion-pair separation between events evaporated from pure GaN and Al0.3Ga0.7N. From this, we aim to define a relative measure for the bond length of ion pairs within an AlGaN/GaN heterostructure. The distributions of pair separation revealed a distinct bimodal behavior that is unique to Al-N2+ ion pairs, suggesting the occurrence of both co-evaporation and molecular dissociation. Finally, we demonstrated that the two modes of ion-pair events align with the known variation in the surface electric field of the AlGaN(0001) structure. These findings demonstrate the utility of atom probe tomography in studying the crystallographic nature of nitride semiconductors.

Correlation of Multiplicity and Chemistry in Al x Ga1-xN Heterostructure via Atom Probe Tomography.

In this work, the correlation between composition and relative evaporation field was investigated by tracking the statistics of multi-hit detector events in atom probe tomography (APT). This approach is applied systematically to a GaN-based nitride heterostructure with five AlxGa1-xN layers of varying Al composition. The relative field evaporation and the percentage of multi-hit events were found to increase with higher Al concentration. Furthermore, the comparison of the relative evaporation fields of AlN with respect to the constituent ions is found to be less than GaN with respect to its constituent ions. Despite equivalent compositions between opposing interfaces of the same AlxGa1-xN interlayer, the rate of change in multiplicity exhibits a consistent asymmetric trend with a steeper slope across the AlxGa1-xN/GaN interface compared to the GaN/AlxGa1-xN interface. The AlxGa1-xN/GaN heterostructure serves as a test structure for exploring field evaporation and neighborhood chemistry, which can be applied to any material chemistry and particularly other nitride systems.