Characterization of III-V semiconductor interfaces by Z-contrast imaging, EELS and CBED

The application of scanning transmission electron microscopy (STEM)-based techniques, atomic number (Z)-contrast imaging, electron energy-loss spectroscopy (EELS) and convergent beam electron diffraction (CBED) allows determination of chemical compositions at internal interfaces of semiconductor heterostructures as well as determination of local crystalline properties such as strain, relaxation effects or ordering with high lateral spatial resolution. Z-contrast images recorded at internal heterostructure interfaces exhibit atomic spatial resolution in combination with qualitative chemical information. EELS can be used to record the chemical composition quantitatively but with slightly decreased spatial resolution compared to Z-contrast imaging. However, EELS results can be used to calibrate the Z-contrast. Thus, the combination of both techniques can give quantitative information on the chemical composition at interfaces from monolayer to monolayer. The interpretation of Z-contrast imaging is further supported by Z-contrast simulations. Examples demonstrating the performance of Z-contrast imaging (and simulation) and EELS are given for technically relevant Ill-V heterostructure interfaces. Additionally, we used CBED in order to investigate the crystalline properties of cross sectional specimens from ternary and quaternary heterostructures of GaxIn1-xAsyP1-y on InP or GaAs substrates. Even when using subnanometer electron probes, the quality of the obtained CBED patterns is sufficient to perform local strain measurements with 1 nm spatial resolution and with a sensitivity of (Delta a/a)(perpendicular to) approximate to 10(-3). This is proved by a CBED linescan across an alternately strained quaternary superlattice. CBED patterns recorded at interfaces directly exhibit symmetry violations, which are not yet understood satisfactorily. Therefore, further simulations are necessary for a detailed quantitative understanding of CBED patterns from internal interfaces. The combination of Z-contrast imaging, EELS and CBED allows the extensive quantitative characterization of semiconductor heterostructures and interfaces with the necessary lateral spatial resolution down to the monolayer range. STEM-based techniques are therefore an important tool for heterostructure and device development.