Setting limits on the accuracy of X-ray determination of al concentration in AlGaAs/GaAs epitaxial layers

A review of the existing literature shows that the accuracy with which the Al composition of a coherent AlxGa1-xAs/GaAs heteroepitaxial layer can be determined from the lattice parameter difference between it and the substrate has been a topic of considerable controversy. The present work uses high resolution X-ray diffraction (HRXRD) and electron probe microanalysis (EPMA) to study 47 samples of epitaxial AlGaAs on commercially available GaAs(SI) substrates over the entire composition range in order to investigate the accuracy and reproducibility which can realistically be achieved. Several factors which can contribute to inaccuracies were revisited experimentally in order to assess their contribution; these included: differences in the lattice parameter of commercially available GaAs(SI) substrates between liquid encapsulated Czochralski (LEC) and vertical gradient freeze (VGF) substrates, as well as boule to boule variations, intrinsic differences in the lattice parameter of AlGaAs prepared by MBE and MOCVD, and diffractometer theta step size variations for three different instruments. The data show that in the x=0.2-0.8 range the composition is a linear function of the angular separation: Delta Theta(004) (arcsec)=-368 x and that the absolute Al compositions can routinely be determined at best to within +/-1% Al. This approach uses neither the absolute lattice parameters of GaAs and AlAs nor their Poisson ratios; however, the spread on the experimental data from this large number of samples was used to calculate upper limits on the range for both. The same samples as discussed above were used to show that, with care, secondary ion mass spectroscopy (SIMS) can be calibrated over a limited composition range to provide a measurement of the Al mole fraction. SIMS measurements, which are insensitive to the substrate lattice parameter, were compared to HRXRD measurements of Al composition for AlGaAs epitaxial layers grown on intentionally doped GaAs substrates (n-type, Si: (1-3)x10(18) cm(-3)). This revealed that substrate doping can have a substantial effect (as much as 3%) on the HRXDX determination of Al composition.