We investigate exciton states theoretically in strained GaN/AlN quantum dots
with wurtzite (WZ) and zinc-blende (ZB) crystal structures, as well as strained
WZ GaN/AlGaN quantum dots. We show that the strain field significantly modifies
the conduction and valence band edges of GaN quantum dots. The piezoelectric
field is found to govern excitonic properties of WZ GaN/AlN quantum dots, while
it has a smaller effect on WZ GaN/AlGaN, and very little effect on ZB GaN/AlN
quantum dots. As a result, the exciton ground state energy in WZ GaN/AlN
quantum dots, with heights larger than 3 nm, exhibits a red shift with respect
to the bulk WZ GaN energy gap. The radiative decay time of the red-shifted
transitions is large and increases almost exponentially from 6.6 ns for quantum
dots with height 3 nm to 1100 ns for the quantum dots with height 4.5 nm. In WZ
GaN/AlGaN quantum dots, both the radiative decay time and its increase with
quantum dot height are smaller than those in WZ GaN/AlN quantum dots. On the
other hand, the radiative decay time in ZB GaN/AlN quantum dots is of the order
of 0.3 ns, and is almost independent of the quantum dot height. Our results are
in good agreement with available experimental data and can be used to optimize
GaN quantum dot parameters for proposed optoelectronic applications.Comment: 18 pages, accepted for publication in the Journal of Applied Physic