New methods for qualitative and quantitative analysis of the GaAs/AlGaAs interface by high-resolution electron microscopy

Abstract

New methods for both qualitative and quantitative analysis of the GaAs/AlGaAs interface by high-resolution electron microscopy at 400 kV in <100& projection are presented. Accurate determination of the microscope defocus and minimizing of both beam and crystal tilt are achieved for cleaved 90° wedge-shaped specimens with the help of nonlinear contrast details. An estimate of the width of the transition region at a diffuse interface is obtained from the shift of the virtual interface with increasing specimen thickness in experimental images of cleaved wedge-shaped specimens. Thickness and defocus ranges optimized for quantitative determination of the composition profile across the interface were found by an extensive study of the nonlinear imaging theory. For images recorded under these optimum imaging conditions a new efficient image-processing algorithm for quantitative composition determination was developed. By this, the chemical composition at interfaces can be measured with near-atomic spatial resolution parallel and perpendicular to the interface. Because our algorithm takes into account only the chemically sensitive Fourier components, noise reduction is obtained compared to earlier suggested algorithms. The influence of beam and crystal tilt is investigated by image simulations. Our algorithm is applied to HREM images of 90° wedge-shaped specimens consisting of a GaAs/AlAs multi-quantum-well structure. Due to the statistical (quantum noise, radiation damage and surface roughness) and systematic errors (limited microscope resolution and beam tilt) composition profiles across the interface can be determined within an uncertainty of Δ x≤0.1 per atomic layer (0.28 nm).