Towards Composition Determination in Compound Semiconductors by High-Angle Annular Dark-Field Imaging

Abstract

An analytical technique with high spatial resolution is required to investigate the composition of thin layers in compound semiconductor multilayer specimens. The techniques currently in use in TEM can broadly be divided into those in which element-specific signals are collected (e.g. EDX) and those in which compositional information is inferred from the variation in the position or magnitude of some contrast feature in the image (e.g. the displacement of thickness fringes in a wedge-shaped sample ). In this paper we report on the determination of the atomic fraction of Al, denoted x, in the Alx Ga1-x As/GaAssystem by analysis of high-angle annular dark field (HADF) images. The principal advantage of using an imaging technique is that the required information is contained in the variation of a signal proportional to an elastic scattering or a thermal-diffuse scattering cross-section rather than one related to an inner shell excitation. As the former are generally a few orders of magnitude larger than the latter it is possible to obtain an image from a wide area of specimen in the time that is required to collect an adequate EDX spectrum from a single position. Thus accumulation of the required information is much more rapid. Instances where it is desirable to perform a quantitative assay over a wide area include specimens in which there are graded interfaces where the atomic fraction varies continuously (e.g. to obtain specific light-guiding properties) or specimens which are deposited at elevated temperatures where Ga desorption can lead to significant variations from the expected composition. In both circumstances it is useful to make measurements at several locations in planes perpendicular to the growth direction. This is possible using techniques where cross-sectional specimens are employed but not where the presence of a wedge is essential.