|СЗМ Раман Нано ИК системы||Модульные СЗМ||Автоматизированные СЗМ||Специализированные СЗМ|
Отображение локальной жесткости.
Local elasticity mode imaging.
In some heterostructure samples, for example hexagonal GaN-based heterostructures epitaxially grown on sapphire substrates, the cleavage plane of epitaxial layers does not coincide with the cleavage plane of the substrate. On the cleavages of such samples the relief height variations may exceed several hundreds nanometers. A fruitful way to reveal different epitaxial layers is to apply local elasticity (force modulation technique) mode. A local elasticity image of the short-period AlGaN(7nm)/GaN(7nm) superlattice is shown in figure. Both buffer GaN layer and the periodical structure of the superlattice are clearly resolved in image. AlGaN corresponds to the higher signal (brighter contrast). Careful examination of the long-period AlGaN/GaN superlattices (20, 40 and 80 nm) demonstrated strong correlation between topography and local elasticity signals [6,7]. Namely, low (high) local elasticity signal was observed at the relief elevations (depressions). Simple analysis shows decreasing of the tip-surface contact stiffness at elevations compared to depressions. We believe, therefore, that the local elasticity signal behavior on short-period superlattice has also mainly topographical origin. The interdependence of the topography relief and heterostructure composition, revealed by the local elasticity mode, has following reasons. Lattice constant of AlGaN is less than that of GaN, therefore GaN layers are compressed, whereas AlGaN layers are tensed in the interface plane. On the free surface the bulk periodic strain variations have to be balanced by the Laplas pressure under convex and concave relief: compressions create the convex relief elevations and tensions do the concave relief depressions .
AFM images are obtained by commercial SPM P4 with CSC12 cantilevers.