Influence of the heterostructure design on the optical properties of GaN and Al_0.1Ga_0.9N quantum dots for ultraviolet emission

The optical properties of Al_yGa_1-yN quantum dots (QDs), with y = 0 or y = 0.1, in an Al_xGa_1−xN matrix are studied. The influence of the QD layer design is investigated pointing out the correlations between the QD structural and optical properties. In a first part, the role of the epitaxial strain in the dot self-assembling process is studied by fabricating GaN QD layers on different Al_xGa_1−xN layers with 0.5 ≤ x ≤ 0.7. Photoluminescence (PL) measurements show the main influence of the increase of the internal electric field (F_int) on the QD optical response inducing a strong red shift in the emission energy as x increases. Time resolved combined with temperature dependent PL measurements enabled the estimation of the QD internal quantum efficiencies at low temperature showing values around 50%. In addition, a PL integrated intensity ratio up to 74% is shown, between 300 and 9 K. In the second part, the design of Al_0.1Ga_0.9N QDs was investigated, by varying the Al_0.1Ga_0.9N amount deposited. An increase of the transition energy (from 3.65 eV up to 3.83 eV) is obtained while decreasing the deposited amount. Calculations of the ground state transition energies as a function of the Al_0.1Ga_0.9N dot height give a value of F_int around 2.0 ± 0.5 MV/cm. Therefore, the propensity of Al_0.1Ga_0.9N dots to emit at much higher energies than GaN dots (a PL shift of ∼1 eV using a low excitation power) is seen as the consequence of the reduced F_int together with their smaller sizes.