Practical Issues for Atom Probe Tomography Analysis of III-Nitride Semiconductor Materials

Abstract

Various practical issues affecting atom probe tomography (APT) analysis of III-nitride semiconductors have been studied as part of an investigation using a c-plane InAlN/GaN heterostructure. Specimen preparation was undertaken using a focused ion beam microscope with a mono-isotopic Ga source. This enabled the unambiguous observation of implantation damage induced by sample preparation. In the reconstructed InAlN layer Ga implantation was demonstrated for the standard "clean-up" voltage (5 kV), but this was significantly reduced by using a lower voltage (e.g., 1 kV). The characteristics of APT data from the desorption maps to the mass spectra and measured chemical compositions were examined within the GaN buffer layer underlying the InAlN layer in both pulsed laser and pulsed voltage modes. The measured Ga content increased monotonically with increasing laser pulse energy and voltage pulse fraction within the examined ranges. The best results were obtained at very low laser energy, with the Ga content close to the expected stoichiometric value for GaN and the associated desorption map showing a clear crystallographic pole structure.

Validity of Vegard’s rule for Al1−xInxN (0.08 < x < 0.28) thin films grown on GaN templates

“…These results are consistent with several studies in the literature reporting parasitic Ga-incorporation in Al 1−x In x N layers where the exact Ga-profiles will depend on the history of previous growth runs in the reactor [35,[55][56][57][58][59][60]. InN content derived from XRD using Vegard's rule [27] or the modified Vegard's rule described by equation (1) [30] as a function of the InN content measured by RBS for sample series C (a), S (b) and T (c).…”

“…Possibly such anomalous behaviour can also be explained by hydrostatic strain introduced by defects or by impurities. In particular, Ga contamination features in the literature on MOCVD growth of Al 1-x In x N films [35,[55][56][57][58][59][60].…”

“…APT reconstruction and analysis were carried out using a CAMECA IVAS ™ software package calibrated by the thicknesses of Al 1−x In x N layers measured by XRD. APT samples were prepared using a dual beam focussed ion beam based lift-out technique (FEI: Helios NanoLab ™ ) [35].…”

“…It is worth noting that APT analysis of the stoichiometry of Table 1. Relaxed lattice parameters of the binaries AlN [42] and InN [43] and stiffness coefficients [43,44] III-nitride materials is significantly dependent on the parameters used in the APT experiment for reasons which are still under debate [35,[49][50][51][52]. Nevertheless, for the analysis of both In 1-y Ga y N and Al 1−x In x N, the measured fraction of metallic sites occupied by In atoms has been found to be relatively stable to the running conditions [53].…”

Removal of Ga Implantation on FIB-prepared Atom Probe Specimens Using Small Beam and Low Energy Ar+ Milling

“…APT specimens are typically prepared using a dual beam focused ion beam (DB-FIB), which is an efficient tool for removing a substantial amount of material, and in situ electron beam imaging allows more control when shaping the APT specimen tip [1]. However, Ga-induced damage and implantation from FIB milling can result in ambiguous results, especially for Al/Al alloys [2] and Ga containing materials [3]. Low energy (< 1 keV) Ar + milling has been shown to improve TEM specimen quality by removing Ga damage and implantation from FIB preparation [4][5].…”