Mechanism for improvements of optical properties of 1.3-μm InAs/GaAs quantum dots by a combined InAlAs-InGaAs cap layer
The optical and structural properties of InAs quantum dots (QDs) with a thin InAlAs-InGaAs composite cap layer have been systematically investigated by photoluminescence and transmission electron microscopy (TEM). A number of improvements in the optical properties are observed with the use of an InAlAs-InGaAs cap layer, instead of InGaAs. These include a redshift of the emission, a reduction of the photoluminescence linewidth, an increased separation between the ground- and first-excited-state transitions, and an enhancement of the photoluminescence intensity at room temperature. To understand these optical improvements, the structural characteristics of the dots are studied by cross-sectional TEM. The height of the QDs is found to increase with increasing InAlAs thickness in the InAlAs-InGaAs cap layer. In addition, scanning TEM is used to qualitatively map the Al distribution in the vicinity of the QDs. These studies indicate that Al atoms are not deposited directly above the QDs in the present structures and hence that the InAlAs cap layer mainly affects the lateral potential barrier of the QDs. The improvements of the QD optical properties can thus be explained in terms of the increased QD height and lateral potential barrier. A consideration of mass transport during the capping process provides a possible growth mechanism responsible for the formation of taller InAs dots when capped with a thin Al-containing layer.