  We investigate sub-monolayer InN quantum sheets embedded in GaN(0001) by
temperature-dependent photoluminescence spectroscopy under both continuous-wave
and pulsed excitation. Both the peak energy and the linewidth of the emission
band associated with the quantum sheets exhibit an anomalous dependence on
temperature indicative of carrier localization. Photoluminescence transients
reveal a power law decay at low temperatures reflecting that the recombining
electrons and holes occupy spatially separate, individual potential minima
reminiscent of conventional (In,Ga)N(0001) quantum wells exhibiting the
characteristic disorder of a random alloy. At elevated temperatures, carrier
delocalization sets in and is accompanied by a thermally activated quenching of
the emission. We ascribe the strong nonradiative recombination to extended
states in the GaN barriers and confirm our assumption by a simple rate-equation
model.
