Deep levels in GaAs(001)/InAs/InGaAs/GaAs self-assembled quantum dot structures and their effect on quantum dot devices

Currently lattice mismatch strain-driven three-dimensional coherent island based quantum dots, dubbed self-assembled quantum dots (SAQDs), constitute the most developed class of quantum dots with successful applications to lasers and considerable potential for infrared detectors in the 1-12 mu m regime. This is in no small part a consequence of the extensive studies on the formation and control of the islands and on their capping by appropriate overlayer materials under optimal growth conditions. By contrast, surprisingly few studies have been reported on the presence and nature of the deep levels in SAQD structures, much less direct studies of the impact of deep levels on SAQD based device characteristics. The latter is of particular significance to devices such as detectors that require large numbers of SAQD layers [i.e., multiple quantum dot (MQD) structures] and are thus increasingly prone to accumulating strain-induced defect formation with increasing numbers of quantum dot layers. In this paper, we report the results of a study of the density, energy profile, and spatial profile of deep levels in different regions of GaAs(001)/InAs/InGaAs/GaAs SAQD structures in which the InGaAs/GaAs capping layers have been grown at different growth conditions. Different types of deep levels are found in different regions and, as expected, their densities are found to increase in the presence of the SAQDs. The study shows that it is the density of deep levels in the GaAs capping layer, forced to be grown at the low temperature of similar to 500 degrees C to suppress In outdiffusion, which has a significant adverse impact on quantum dot device characteristics. Their density can be reduced by growth conditions such as migration enhanced epitaxy that permit high quality overgrowths at temperatures as low as similar to 350 degrees C. Nevertheless, the ultimate performance limitation of thick MQD based devices resides in the ability to realize low density of the deep levels relative to the density of SAQDs. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3359704]