Well-width dependence of the quantum efficiencies of GaN/Al_xGa_1-xN multiple quantum wells

A set of GaN/Al_xGa_1-xN(x≈0.2) multiple quantum wells (MQWs) with well widths, L_w, varying from 6 to 48 Å has been grown by metalorganic chemical vapor deposition under the optimal GaN-like growth conditions. Picosecond time-resolved photoluminescence spectroscopy has been employed to probe the well-width dependence of the quantum efficiencies (QE) of these MQWs. Our results have shown that these GaN/AlGaN MQW structures exhibit negligibly small piezoelectric effects and hence enhanced QE. Furthermore, GaN/Al_xGa_1-xN MQWs with L_w between 12 and 42 Å were observed to provide the highest QE, which can be attributed to the reduced nonradiative recombination rate as well as the improved quantum-well quality. The decreased QE in GaN/Al_xGa_1-xN MQWs with L_w< 12 Å is due to the enhanced carrier leakage to the underlying GaN epilayers, while the decreased QE in MQWs with L_w> 42 Å is associated with an increased nonradiative recombination rate as L_w approaching the critical thickness of MQWs. The implications of our results on device applications are also discussed.