Photoluminescence (PL) from InAs self-assembled quantum dots (QD) embedded in the AlAs matrix was strong and clean around 700 nm. PL efficiency remained quite high at room temperature compared to other QD systems embedded in GaAs cladding layers. Transmission electron microscope pictures from the structure showed a clear formation of relatively small and coherently strained InAs QD. The observed blueshift with accompanying broadening of PL spectra with the increase of excitation power is interpreted in terms of local carrier tunneling in a dense QD system. The PL peak redshift with the increase of temperature was very large, as much as 228 meV. The anomalous shift is interpreted as due to activation-energy differences between dots of different sizes.
The fabrication of nanometer-scale GaAs dots on AlGaAs layer by molecular beam epitaxy was
demonstrated. Unlike the stress-driven transition of the three-dimensional growth mode in the
lattice-mismatched system, the limited migration of Ga droplets on the AlGaAs
layer grown at low substrate temperature was exploited to give rise to the formation of
three-dimensional GaAs islands. The resulting GaAs dots show crater-like features
having {111} facets. In micro-photoluminescence measurements of the
buried structures, the emission spectra were clearly observed, and the
sharp lines of the spectra might be considered as the exciton
emissions from individual dots with various sizes.
We have found that the level spacing between the ground and first excited states of InAs∕GaAs quantum dots (QDs) increases as the photoluminescence peak energy decreases, that is, as the QD increases in size. By means of simple numerical calculations, we confirm that this seemingly unusual level-spacing behavior originates from the low aspect ratio of typical QDs with a finite potential barrier. Carrier lifetime measurements show that QDs with a lower photoluminescence peak energy tend to have a shorter decay time, which can be attributed to better confinement of the electron wave function and the resultant increase in electron-hole wave function overlap.
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