Low-temperature growth of InAs nanostructures on GaAs by MBE.
A.F.Ioffe Physico-Technical Institute, Russian Academy of Sciences
Politekhnicheskaya 26, 194021 St.Petersburg, Russia
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Stranski-Krastanow quantum dots (QDs) are currently considered as promising candidates for the use in active regions of semiconductor optoelectronic devices with advanced characteristics. Therefore, it is very important to have a possibility of controlling the main properties of QDs during the MBE deposition, like size and shape, which determine the emission wavelength.
It is well known that the substrate temperature strongly affects not only size and shape of InAs quantum dots but also the surface density of a quantum dot ensemble. The increase of the substrate temperature leads to the decrease in aspect ratio, increase in base length, and decrease in surface density.
We have recently shown that the low-temperature (300-350° C) deposition of InAs QDs leads to the formation of agglomerates of QDs which are the origin of unusual long-wavelength emission at 1.6-1.7 m m [1,2]. However, the intensity of this luminescence line was low and insufficient for the use of these objects in optoelectronic devices. We have found that the reason for this was the high density of defects and dislocations which deteriorate the luminescence characteristics.
PL and PLE studies show QD-like nature of the long wavelength emission. The linear increase of integral intensity of photoluminescence with excitation density up to high excitation densities, typical for QD structures shape of the PLE spectra, and significant intensity of the long wavelength emission under resonant excitation with a photon energy below the GaAs band gap entirely exclude defects formed in the low temperature thin GaAs layer as the origin of the 1.6-1.7 m m luminescence.
Using photoluminescence and transmission electron microscopy we show that special in-situ procedure including the overgrowth of InAs QDs with thin GaAs layer followed by the short increase of the substrate temperature allows us to increase significantly the intensity of the long wavelength line and the reason for this is the elimination of defects. The PL intensity strongly depends on the thickness of the GaAs cap layer. Thick cap layer only slightly affects the PL intensity, the use of too thin layer leads to almost complete disappearance of the long wavelength line and only some intermediate thickness considerably increases the PL intensity due to the decrease in the defect density. Strong increase in the anneal temperature show no pronounced effect on the PL intensity but can result in the QD-GaAs intermixing.
To conclude, optimization of the growth regimes by MBE can lead to significant improvement of the long wavelength QD emission. Further investigations could lead to the use of InAs QDs for optoelectronic devices in the practically important 1.55 m m wavelength region.