  Silicon (Si) donors in GaAs have been the topic of extensive studies since Si
is the most common and well understood n-type dopant in III-V semiconductor
devices and substrates. The indirect bandgap of AlAs compared to the direct one
of GaAs leads to interesting effects when introducing Si dopants. Here we
present a study of cross-sectional scanning tunneling microscopy (X-STM) and
density functional theory (DFT) calculations to study Si donors in AlAs at the
atomic scale. Based on their crystal symmetry and contrast strengths, we
identify Si donors up to four layers below the (110) surface of AlAs.
Interestingly, their short-range local density of states (LDOS) is very similar
to Si atoms in the (110) surface of GaAs. Additionally we show high-resolution
images of Si donors in all these layers. For empty state imaging, the
experimental and simulated STM images based on DFT show excellent agreement for
Si donor up to two layers below the surface.
