Excitonic properties of strained wurtzite and zinc-blende GaN/AlxGa1−xN quantum dots

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