Photoluminescence spectroscopy study of excited states in InxGa1-xAs-capped InAs quantum dots

This study reports on photoluminescence (PL) and PL excitation (PLE) spectroscopy studies of excited state transitions in InxGa1-xAs-capped InAs quantum dots (QDs). InAs self-assembled QDs were grown on GaAs (001) wafers using 2.5 mono layer (ML) InAs deposition via molecular beam epitaxy. GaAs-capped QDs showed PL peak position at 1.182 mu m at 78 K with a narrow full width at half maximum of 23 meV, indicating a highly uniform QD ensemble. The PLE spectrum of GaAs-capped QDs showed four peaks related to the QD excited state transitions at 1.117, 1.140, 1.192 and 1.224 eV at 8 K. Both ground state and excited state interband transitions were red-shifted when the first 30 ML of GaAs capping layer were replaced by InxGa1-xAs layer. The PL peak of In0.2Ga0.8As-capped QDs reached 1.246 mu m (1.335 mu m) at 78 K (296 K). Energy differences between the excited and ground states became smaller with increasing In composition of the InxGa1-xAs capping layers because of increasingly lower InAs to InxGa1-xAs band edge discontinuity as a result of the chemical and strain relieving effects. In addition, PLE spectra clearly showed wetting layer and quantum well states caused by the InxGa1-xAs capping layer for GaAs- and InxGa1-xAs-capped QDs, respectively. Based on the current PL, PLE, and the calculated transition energies available in literature, the lowest electron state levels of similar to 60 meV and similar to 112 meV above the electron ground state and the lowest hole state levels of similar to 23 meV and similar to 55 meV above the hole ground state were extracted.