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

Abstract

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 μm 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 ∼500 °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 ∼350 °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.

Impact of nanostructures and radiation environment on defect levels in III–V solar cells

“…This EL2-like defect has been previously seen by other groups in samples containing InAs QDs [13,18,19]. Asano et al has shown that this defect may be due in part to the low temperature GaAs capping layer, used to stabilize the InAs surface before a temperature ramp, rather than the QD itself [19]. Since QD-EL2 it is near the midgap and has a density in the 10 14 cm -3 range, it would likely be an effective majority or minority carrier trap, and thus could have a larger influence on minority carrier device properties (e.g.…”

“…The QD-EL2 defect has a relatively large capture cross-section (suggesting a complex defect similar to EL2), is near the mid- gap, and has a relatively high defect density. This EL2-like defect has been previously seen by other groups in samples containing InAs QDs [13,18,19]. Asano et al has shown that this defect may be due in part to the low temperature GaAs capping layer, used to stabilize the InAs surface before a temperature ramp, rather than the QD itself [19].…”

Investigation of electrically active defects in InGaAs quantum wire intermediate-band solar cells using deep-level transient spectroscopy technique

“…Furthermore, E2 PIN and E3 QWR have comparable activation energies as trap F (0.14 eV) reported by Asano et al [48] in GaAs (001) /InAs/InGaAs/GaAs self-assembled QD structures. In their study, they inferred that the increase of the density of this trap and others traps around the QDs is due to the growth conditions of InGaAs/GaAs QD structures.…”

Photoelectric properties of the metamorphic InAs/InGaAs quantum dot structure at room temperature

“…Considering the data about intrinsic and crystal defects in In(Ga)As/GaAs heterostructures obtained from deep level transient spectroscopy, 23,24 PC spectroscopy, and thermally stimulated current, 16,17,25 we attribute the onset of the absorption spectrum at 0.68 eV to a transition from a EL2 defect center in the InGaAs/GaAs layer where defect concentration is high. 26,27 It is known that in semi-insulating GaAs, the level of EL2 center is located near 0.75 eV below the conduction band, 28 but in strained InGaAs the energy of transition involving EL2 defect states is reduced.…”