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Abstract

The dynamics of free and bound excitons and their interactions
have been analyzed from the results of time-resolved
photoluminescence (PL) in GaN layers fabricated by metalorganic
chemical vapor deposition on GaN single-crystal substrates. The
PL transients of an acceptor-bound exciton (A(0)X) a donor-
bound exciton ((DX)-X-0), and free excitons A and B (FXA and
FXB) at 3.467 eV, 3.472 eV, 3.478 eV, and 3.483 eV,
respectively, have been measured. At low excitation density
(100 W/cm(2)), the PL decay times in GaN range from 30 ps for
FXB up to 1000 ps for A(0)X. The dependence of time constants
versus excitation and temperature has been analyzed. Below 50
K, the most important decay process of the FX is trapping on
donors. At higher temperatures, delocalization of excitons is
observed, and free-exciton emission becomes relatively strong.
The delocalization of excitons influences the decay process.
Consequently, decay times initially increase, then decrease
with temperature. They are longest at about T=70 K. The time
constants of the FXA and (DX)-X-0 are of the order of a few
hundred ps. The exciton temperature T-X has been determined
from the slope of the blue wing of the FX peak. Initially, the
excitons are hot (due to high photon energy); then they cool
due to interactions with phonons on a time scale of about 80
ps. In the same time range, a (DX)-X-0-FX interaction leading
to a broadening of the (DX)-X-0 peak is observed.