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

TL;DR

This study uses laser-assisted atom probe tomography to measure indium distribution in InGaN structures, comparing results with RBS, and investigates effects of laser pulse energy, sample orientation, and electric field on analysis accuracy and tip morphology.

Contribution

It demonstrates optimized APT conditions for accurate indium quantification in InGaN and reveals asymmetries and electric field effects in m-plane and c-plane samples.

Findings

APT results agree with RBS at specific laser energies

Charge-state ratio asymmetry affects tip shape in m-plane samples

Electric field strength is influenced by p-type inversion layers

Abstract

Laser-assisted atom probe tomography (APT) was used to measure the indium mole fraction x of c-plane, MOCVD-grown, GaN/In(x)Ga(1-x)N/GaN test structures and the results were compared with Rutherford backscattering analysis (RBS). Four sample types were examined with (RBS determined) x = 0.030, 0.034, 0.056, and 0.112. The respective In(x)Ga(1-x)N layer thicknesses were 330 nm, 327 nm, 360 nm, and 55 nm. APT data were collected at (fixed) laser pulse energy (PE) selected within the range of (2-1000) fJ. Sample temperatures were = 54 K. PE within (2-50) fJ yielded x values that agreed with RBS (within uncertainty) and were comparatively insensitive to region-of-interest (ROI) geometry and orientation. By contrast, approximate stoichiometry was only found in the GaN portions of the samples provided PE was within (5-20) fJ and the analyses were confined to cylindrical ROIs (of diameters =20 nm) that were coaxial with the specimen tips. m-plane oriented tips were derived from c-axis grown, core-shell, GaN/In(x)Ga(1-x)N nanorod heterostructures. Compositional analysis along [0 0 0 1] (transverse to the long axis of the tip), of these m-plane samples revealed a spatial asymmetry in charge-state ratio (CSR) and a corresponding asymmetry in the resultant tip shape along this direction; no asymmetry in CSR or tip shape was observed for analysis along [-1 2-1 0]. Simulations revealed that the electric field strength at the tip apex was dominated by the presence of a p-type inversion layer, which developed under typical tip-electrode bias conditions for the n-type doping levels considered. Finally, both c-plane and m-plane sample types showed depth-dependent variations in absolute ion counts that depended upon ROI placement.