Laser-assisted atom probe tomography of c-plane and m-plane InGaN test structures

Abstract

Laser-assisted atom probe tomography (L-APT) was used to measure the indium mole fraction x of c-plane, MOCVD-grown, GaN/InxGa1-xN/GaN test structures and the results were compared with Rutherford backscattering analysis (RBS). Four separate sample types were examined with x = 0.030, 0.034, 0.056, and 0.11 as determined by RBS. The respective InxGa1-xN layer thicknesses were 330 nm, 327 nm, 360 nm, and 55 nm. L-APT data were collected under (fixed) laser pulse energy (PE)conditions in the range of (2 1000) fJ. Sample temperatures were generally 54 K. The data were analyzed using different region-of-interest (ROI) volumes to establish criteria for both data collection and compositional analysis that would return L-APT results that fell within the estimated 0.5 at. % uncertainty of RBS. Using cylindrical ROIs placed coaxially within the reconstructed InxGa1-xN regions, and with ROI depths of (55 180) nm, PE values in the range of(2 100) fJ yielded indium concentrations that conformed to RBS. L-APT results were comparatively insensitive to ROI diameter such that 20-nm-diameter coaxial ROIs yielded indium concentrations nearly equal to ROIs that encompassed the full InxGa1-xN regions. By contrast, for the GaN portions of the samples, L-APT would only yield near-stochiometric composition for PE in the range of (5 20) fJ with the analysis restricted to coaxial ROIs of diameters ?20 nm. m-plane oriented L-APT specimens were derived from core-shell GaN/InxGa1-xN multi-quantum-well structures. Analysis within ROIs placed along [0001] of these m-plane samples revealed a spatial asymmetry in charge-state ratios that is consistent with an electrostatic effect arising from spontaneous polarization. Both c-plane and m-plane sample types showed depth-dependent variations in absolute ion counts that depended upon ROI placement.