Surface relaxation of strained Ga(P,As)/GaP heterostructures investigated by HAADF STEM

• Published in: Journal of microscopy (Oxford) 2017-12, Vol.268 (3), p.239-247
• Main Authors: BEYER, ANDREAS, DUSCHEK, LENNART, BELZ, JÜRGEN, OELERICH, JAN OLIVER, JANDIERI, KAKHABER, VOLZ, KERSTIN
• Format: Article
• Language: English
• Subjects: Atomic resolution; Columns (structural); Crystal lattices; Diffraction patterns; Electron diffraction; Electrons; Finite element method; frozen phonon simulations; Heterostructures; High resolution; Image resolution; Planes; Quantum wells; Scanning transmission electron microscopy; Semiconductor devices; Simulation; strained interfaces; surface relaxation; Thin films; Transmission electron microscopy; Variations
• ISSN: 0022-2720; 1365-2818
• DOI: 10.1111/jmi.12622

Summary The surfaces of thin transmission electron microscopy (TEM) specimens of strained heterostructures can relax. The resulting bending of the lattice planes significantly influences high‐angle annular dark field (HAADF) measurements. We investigate the impact by evaluating the intensities measured at the atomic columns as well as their positions in high‐resolution HAADF images. In addition, the consequences in the diffraction plane will be addressed by simulated position averaged convergent beam electron diffraction (PACBED) patterns. The experimental column intensities and positions acquired from a strained Ga(P,As) quantum well (QW) embedded in a in a GaP matrix agree very well with frozen phonon contrast simulations, if the surface relaxation is taken into account by finite element relaxation. Neglecting the surface relaxation the As content of the QW can be significantly underestimated. Taking the effects into account correctly, we find that the lower interface of the investigated Ga(P,As) QW is atomically abrupt whereas the upper one is smeared out. Lay description Modern semiconductor devices usually consist of multiple layers of different materials. Scanning transmission electron microscopy (STEM) is one of the leading techniques in the characterisation of such structures. When a thin TEM sample of such a multilayer is prepared, the surface of the sample can change its geometry, due to relaxation of the strain at the interface between the different materials. Because of this surface relaxation, the lattice planes in the crystal are now longer perfectly straight but they bend. This bending significantly influences the intensity in high‐resolution STEM measurements. In this manuscript we highlight the occurring effects in experimental STEM images of thin Ga(P,As) layers grown on GaP substrate. The contrast simulations which take into account the surface relaxation are in very good agreement with the experimental data. In contrast, when the surface relaxation is ignored, the data can be easily misinterpreted as a fluctuation of the chemical composition across the layer. For the investigated sample, we find that the lower interface is atomically abrupt, whereas the upper interface is smeared out.