Atomic structure, strain and chemical composition at the nanometric scale in Ge/Si quantum effect devices

Abstract

Germanium quantum dots were grown by ultra-high vacuum chemical vapour deposition on {100} silicon substrates. Two types of heterostructures were studied: single islands randomly grown on silicon as well as stacking islands grown in between silicon spacers. {110} cross-sections were prepared by mechanical polishing for high resolution electron microscopy observation (HREM) and X-ray nanoanalysis with a 200-kV transmission electron microscope equipped with a field emission gun. Two methods were considered for measuring the local displacements from the HREM images. The first consists of localizing the centre of the atomic columns with sub-pixel resolution by image processing mostly in real space. In the second method, the displacements were measured through the local Fourier components of the lattice fringes, by calculating two-phase images from two reciprocal vectors. Strains, stresses and germanium concentrations were then deduced from the displacements. Only calculations leading to comparable results from both methods were retained. Quantitative X-ray nanoanalysis was carried out by refining the Cliff–Lorimer factors for Ge with a tri-layered template calibrated by measuring the different Ge concentrations via Rutherford back scattering. A comparison of these complementary data leads to a well-documented and quantitative description of the atomic structure, the local displacements and the chemical composition in the heterostructures, and reveals where interdiffusion occurs.

Acknowledgements

We are very grateful to D. Bouchier and Lam H. Nguyen (Institut d’Electronique Fondamentale, Orsay) for the UHV-CVD growth of the SiGe quantum dots.