Theoretical and experimental limits of the analysis of III/V semiconductors using EELS

The optimisation of acquisition conditions for EELS spectroscopy of AlxGa1- xAs heterostructures permits one to find the absolute concentration with a precision of better than Delta = +/-0.02 and to detect changes in concentra tion of +/-0.01 for x = 0 - 0.5. In order to achieve this concentration pre cision, we investigated ways to reduce the influence of three major sources on the inaccuracies of the measurement: the effect of electron channelling which biases the ionisation probabilities on the different atomic sites, t he contribution of the sample surface layers and the accuracy in the spectr al analysis. An optimal specimen orientation that maximises the stability o f the electron densities on Al and As sites without introducing an unaccept able loss of spatial resolution due to sample tilt is found by computing th e electron channelling intensity as a function of the specimen tilt angle. The influence of a Ga enriched surface layer on the analysis is demonstrate d. Two methods for the extraction of the edge intensity from the spectra ar e compared. These methods are shown to give the upper and the lower limit o f the Ga concentration. (C) 2000 Published by Elsevier Science Ltd. All rig hts reserved.