  Recent advances in two-dimensional (2D) crystals make it possible to realize
an ideal interface structure that is required for device applications.
Specifically, a p-n junction made of 2D crystals is predicted to exhibit an
atomically well-defined interface that will lead to high device performance.
Using angle-resolved photoemission spectroscopy, a simple surface treatment was
shown to allow the possible formation of such an interface. Ta adsorption on
the surface of a p-doped SnSe shifts the valence band maximum towards higher
binding energy due to the charge transfer from Ta to SnSe that is highly
localized at the surface due to the layered structure of SnSe. As a result, the
charge carriers of the surface are changed from holes of its bulk
characteristics to electrons, while the bulk remains as a p-type semiconductor.
This observation suggests that the well-defined interface of a p-n junction
with an atomically thin {\it n}-region is formed between Ta-adsorbed surface
and bulk.
