HREM at orthogonal projections of GaAs islands on silicon

Abstract

HREM studies typically examine only one projection of a structure and information in the electron beam direction is lost. In most cases, the structure in this direction is uniform and already known, but in others a second projection needs to be observed. This could involve preparing a second specimen sectioned at right angles to the first, or as described here, tilting a specimen through ±45° and observing the same volume in orthogonal projections. The specimen used here was of GaAs islands on <001> silicon, examined in the Atomic Resolution Microscope at LBL.In the effort to grow defect-free GaAs films on silicon substrates, understanding of the early stages of growth is required. For this experiment we chose a specimen of ultra-thin GaAs grown on silicon substrate by metal-organic chemical vapor deposition (MOCVD). The GaAs was grown at 550°C for a nominal thickness of20nm from trimethyl gallium and arsine sources.The growth conditions lead to three-dimensional growth of islands bound by {111}A planes, terminated on Ga layers.