Inderscience Publishers

Photoluminescence spectroscopy study of excited states in In x Ga 1–x As-capped InAs quantum dots

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This study reports on photoluminescence (PL) and PL excitation (PLE) spectroscopy studies of excited state transitions in In
x
Ga
1–x
As-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 μm 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 In
x
Ga
1–x
As layer. The PL peak of In
0.2
Ga
0.8
As-capped QDs reached 1.246 μm (1.335 μm) at 78 K (296 K). Energy differences between the excited and ground states became smaller with increasing In composition of the In
x
Ga
1–x
As capping layers because of increasingly lower InAs to In
x
Ga
1–x
As 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 In
x
Ga
1–x
As capping layer for GaAs- and In
x
Ga
1–x
As-capped QDs, respectively. Based on the current PL, PLE, and the calculated transition energies available in literature, the lowest electron state levels of ~60 meV and ~112 meV above the electron ground state and the lowest hole state levels of ~23 meV and ~55 meV above the hole ground state were extracted.

Keywords: quantum dots, photoluminescence, InAs, MBE, molecular beam epitaxy, PLE, PL excitation

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