  Deep ultraviolet (UV) optical emission below 250 nm (~5 eV) in semiconductors
is traditionally obtained from high aluminum containing AlGaN alloy quantum
wells. It is shown here that high-quality epitaxial ultrathin binary GaN
quantum disks embedded in an AlN matrix can produce efficient optical emission
in the 219-235 nm (~5.7 to 5.3 eV) spectral range, far above the bulk bandgap
(3.4 eV) of GaN. The quantum confinement energy in these heterostructures is
larger than the bandgaps of traditional semiconductors, made possible by the
large band offsets. These MBE-grown extreme quantum-confinement GaN/AlN
heterostructures exhibit internal quantum efficiency as high as 40% at
wavelengths as short as 219 nm. These observations, together with the ability
to engineer the interband optical matrix elements to control the direction of
photon emission in such new binary quantum disk active regions offers unique
advantages over alloy AlGaN quantum well counterparts for the realization of
deep-UV light-emitting diodes and lasers.
