Atomic Size Mismatch Strain Induced Reversed ADF-STEM Image Contrast Between dilute Semiconductor Heteroepitaxial Layers and Substrates

Abstract

The annular dark field (ADF) image contrast of heteroepitaxial dilute nitride GaN x As 1-x (x = 0.029 and 0.045) layers on GaAs and dilute carbide Si 1-y C y (y <= 0.015) layers on Si were studied with a scanning transmission electron microscope (STEM). Contradictory to the compositional contrast prediction of ADF-STEM image intensity, the lower average atomic number heteroepitaxial strained layers GaN x As 1-x and Si 1-y C y are brighter than the higher average atomic number Si and GaAs substrates for ADF detector semiangle up to 92 mrad. This reversed contrast is due to the localized strain resulting from the difference in atomic size between the substitutional atoms (N, C) and host atoms (GaAs, Si). The application of the reversed ADF-STEM image contrast is discussed in relation to the evaluation of very small amount of substitutional atom compositions in dilute systems.