We present a detailed study, both structural and optical, of GaAs/InAs
/GaAs heterostructures for InAs nominal coverages (L) ranging from 0.6
to 3 ML. Planar transmission electron microscopy (TEM) provides direc
t evidence of the presence of InAs quantum dots (QDs) for all values o
f L, with an increase in their density at high values of L. Transverse
TEM shows also that those QDs have mostly small base angles. Accordin
gly, the evolution of the optical properties of InAs/GaAs is investiga
ted by photoluminescence (PL) and PL excitation measurements (PLE). A
broad PL band is observed in all samples, which is ascribed to the rec
ombination of heavy-hole excitons in the InAs quantum dots, observed w
ith TEM: For thin coverages (L less than or equal to 1.6ML), a narrow
PL band is also observed, which is attributed to recombination of heav
y-hole excitons in a two-dimensional (2D) InAs layer. The two bands sh
ift to lower energy for increasing L. For L greater than or equal to 1
.6 ML, the QD band has a faster shift and exhibits a complex structure
, while the exciton recombination in the 2D-InAs layer vanishes. Those
features, as well as the PLE results, indicate that: (a) quantum dots
are connected by a two-dimensional InAs layer, at least for thin InAs
coverages, which allows an efficient carrier capture into the dots; (
b) the dot size increases with L, the increase being faster for L grea
ter than or equal to 1.6 ML because above this thickness the growth be
comes completely three dimensional. A simple model explains the PL dat
a and results in a dot geometry in agreement with the TEM measurements
. (C) 1998 American Institute of Physics.