Quantification of matrix and impurity elements in Al_xGa_1−xN compounds by secondary ion mass spectrometry

The authors describe a comprehensive secondary ion mass spectrometry (SIMS) calibration procedure for the quantification of matrix and impurity elements of epitaxially grown Al_xGa_1−xN layers over the full compositional range of 0 ≤ x ≤ 1. For that a set of eight samples was grown by metalorganic vapor phase epitaxy, characterized with respect to AlN mole fraction and implanted with impurity and dopant elements (H, C, O, and Si). The compositional analysis using various techniques yielded consistent Al contents x with an accuracy of ±1%. For the quantitative characterization of impurities by SIMS, calibration curves were generated using a 14.5 keV Cs⁺ primary beam at an angle of incidence of 25°. Measured sputter rates decrease with a nearly linear slope as a function of Al content in the range of 0 ≤ x < 0.48. At higher Al concentrations the sputter rates show only a weak dependence on AlN mole fraction. Matrix ion intensity ratios of AlCs⁺/GaCs⁺ change linearly with direct and inverse proportionality as a function of x/(1−x). The absolute sensitivity factors for H, C, and Si follow an exponential reduction with increasing AlN mole fraction only for lower Al concentrations (0 ≤ x < 0.48). The calculated relative sensitivity factors are determined by the respective reference intensities depending on the AlN mole fraction.