DETECTION OF RANDOM ALLOY FLUCTUATIONS IN HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROGRAPHS OF A1GAAS

We have quantitatively analysed high-resolution transmission electron micrographs of GaAs/AlGaAs quantum wells from cleaved specimens. A careful statistical analysis of the pattern of the image unit cells reveals: (1) a significant broadening of the distribution of the chemical signals obtained for AlGaAs compared to the corresponding distribution for GaAs, and (2) a periodicity in the AlGaAs histogram only which it will be shown corresponds to the signal expected for the exchange of single Al and Ga atoms in the atomic columns of the specimen. We show that these effects are in good agreement with binomial statistics and quantitative image simulations. Possible effects of surface contamination and electron beam damage of GaAs and AlGaAs are excluded as explanations. We therefore conclude that we have directly visualized in our high-resolution images random alloy fluctuations in the ternary alloy.