Optical and transport properties of short period InAs/GaAs superlattices near quantum dot formation

We have investigated the optical and transport properties of MBE grown short-period superlattices of InAs/GaAs with different numbers of periods (3 <= N <= 24) and a total thickness 14 nm. Bandstructure calculations show that these superlattices represent a quantum well with average composition In_0.16Ga_0.84As. The electron wave functions are only slightly modulated by the superlattice potential as compared to a single quantum well with the same composition, which was grown as a reference sample. The photoluminescence, the resistance, the Shubnikov-de Haas effect and the Hall effect have been measured as a function of the InAs layer thickness Q in the range 0.33 <= Q <= 2.7 monolayers (ML). The electron densities range from 6.8 to 11.5x10^11 cm^-2 for Q <= 2.0 ML. The photoluminescence and magnetotransport data show that only one subband is occupied. When Q >= 2.7 ML quantum dots are formed and the metallic type of conductivity changes to variable range hopping conductivity.