Surfactant effect of bismuth in the MOVPE growth of the InAs quantum dots on GaAs

We report on the role of bismuth as a surfactant in the growth of InAs quantum dots (QDs) on GaAs (001) by metal organic vapour phase epitaxy. Atomic force microscopy investigations have shown that bismuth suppresses coalescence of the InAs QDs and advances a more uniform size distribution. The photoluminescence spectra of the Bi-assisted grown QDs present several narrow peaks from the ground and the excited state transitions with full width at half maximum (FWHM) as narrow as 25 meV (both at 77 and 300 K). Due to such low values of the FWHM we were able to observe up to two well resolved excited state transitions in the photovoltage spectra measured by an electrolyte cell technique. The lowest ground transition energies observed were 0.93 eV at 77 K and 0.875 eV at 300 K (emission wavelength 1.46 µm). So using Bi-assisted growth it is possible to cover the 1.3 µm band, which is important for optoelectronic applications in the InAs/GaAs material system. Formation of such `deep' QDs without misfit dislocations was explained by the formation of a graded-composition transient InGaAs alloy layer at the GaAs/InAs hetero-interface as a result of diffusion intermixing of the components. The proposed mechanism for the effect of Bi on the QDs' morphology is that Bi decreases the surface mobility of the In atoms on the growing surface, preventing the coalescence of the QDs. Because of its rather large covalent radius (compared with that of As), Bi is not incorporated into the QDs' material segregating on the growing surface.