Growth and properties of the GaN cap layer strongly influenced by the composition of the underlying AlGaN

Abstract

The growth and properties of the GaN cap layers crystallized by metalorganic vapor phase epitaxy were studied concerning the composition of the underlying AlGaN layer, type of induced strains and the desorption of the GaN during the initial cooling step after the epitaxy process. The composition of the underlying AlGaN layers was controlled by the growth conditions, while the type of the stresses (tensile or compressive) was induced by applying GaN or AlN buffer layer respectively. Structures with different thicknesses of the GaN cap layer have been grown and analyzed. Three series of samples containing AlGaN underlayers with different aluminium content (20%, 40% and 60%) were grown. The influence of those underlayers composition and type of induced surface strains on the growth of the GaN cap was discussed. High resolution X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscopy, as well as in-situ reflectometry, were used for sample analysis. It has been shown, that the type of surface strains on top of the AlGaN layer and GaN-AlGaN lattice mismatch strongly influence the final cap layer thickness. It was also confirmed, that a significant reduction of the cap thickness causes its discontinuity, leads to the removal of gallium from the top part of the AlGaN layer by evaporation and thus leads to the formation of a thin aluminium-rich layer at the AlGaN/GaN interface.