  We explore an alternative way to fabricate (In,Ga)N/GaN short-period
superlattices on GaN(0001) by plasma-assisted molecular beam epitaxy. We
exploit the existence of an In adsorbate structure manifesting itself by a
$(\sqrt{3}\times\!\sqrt{3})\text{R}30^{\circ}$ surface reconstruction observed
in-situ by reflection high-energy electron diffraction. This In adlayer
accommodates a maximum of 1/3 monolayer of In on the GaN surface and, under
suitable conditions, can be embedded into GaN to form an
In$_{0.33}$Ga$_{0.67}$N quantum sheet whose width is naturally limited to a
single monolayer. Periodically inserting these quantum sheets, we synthesize
(In,Ga)N/GaN short-period superlattices with abrupt interfaces and high
periodicity as demonstrated by x-ray diffractometry and scanning transmission
electron microscopy. The embedded quantum sheets are found to consist of single
monolayers with an In content of 0.25-0.29. For a barrier thickness of 6
monolayers, the superlattice gives rise to a photoluminescence band at 3.16 eV,
close to the theoretically predicted values for these structures.
