Quantitative analysis of the interplay between InAs quantum dots and wetting layer during the GaAs capping process

Exploring the Implementation of GaAsBi Alloys as Strain-Reducing Layers in InAs/GaAs Quantum Dots

This paper investigates the effect of GaAsBi strain reduction layers (SRLs) on InAs QDs with different Bi fluxes to achieve nanostructures with improved temperature stability. The SRLs are grown at a lower temperature (370 °C) than the usual capping temperature for InAs QDs (510 °C). The study finds that GaAs capping at low temperatures reduces QD decomposition and leads to larger pyramidal dots but also increases the threading dislocation (TD) density. When adding Bi to the capping layer, a significant reduction in TD density is observed, but unexpected structural changes also occur. Increasing the Bi flux does not increase the Bi content but rather the layer thickness. The maximum Bi content for all layers is 2.4%. A higher Bi flux causes earlier Bi incorporation, along with the formation of an additional InGaAs layer above the GaAsBi layer due to In segregation from QD erosion. Additionally, the implementation of GaAsBi SRLs results in smaller dots due to enhanced QD decomposition, which is contrary to the expected function of an SRL. No droplets were detected on the surface of any sample, but we did observe regions of horizontal nanowires within the epilayers for the Bi-rich samples, indicating nanoparticle formation.

Tailoring of AlAs/InAs/GaAs QDs Nanostructures via Capping Growth Rate

The use of thin AlA capping layers (CLs) on InAs quantum dots (QDs) has recently received considerable attention due to improved photovoltaic performance in QD solar cells. However, there is little data on the structural changes that occur during capping and their relation to different growth conditions. In this work, we studied the effect of AlA capping growth rate (CGR) on the structural features of InAs QDs in terms of shape, size, density, and average content. As will be shown, there are notable differences in the characteristics of the QDs upon changing CGR. The Al distribution analysis in the CL around the QDs was revealed to be the key. On the one hand, for the lowest CGR, Al has a homogeneous distribution over the entire surface, but there is a large thickening of the CL on the sides of the QD. As a result, the QDs are lower, lenticular in shape, but richer in In. On the other hand, for the higher CGRs, Al accumulates preferentially around the QD but with a more uniform thickness, resulting in taller QDs, which progressively adopt a truncated pyramidal shape. Surprisingly, intermediate CGRs do not improve either of these behaviors, resulting in less enriched QDs.

Suppressing the Effect of the Wetting Layer through AlAs Capping in InAs/GaAs QD Structures for Solar Cells Applications

Recently, thin AlAs capping layers (CLs) on InAs quantum dot solar cells (QDSCs) have been shown to yield better photovoltaic efficiency compared to traditional QDSCs. Although it has been proposed that this improvement is due to the suppression of the capture of photogenerated carriers through the wetting layer (WL) states by a de-wetting process, the mechanisms that operate during this process are not clear. In this work, a structural analysis of the WL characteristics in the AlAs/InAs QD system with different CL-thickness has been made by scanning transmission electron microscopy techniques. First, an exponential decline of the amount of InAs in the WL with the CL thickness increase has been found, far from a complete elimination of the WL. Instead, this reduction is linked to a higher shield effect against QD decomposition. Second, there is no compositional separation between the WL and CL, but rather single layer with a variable content of InAlGaAs. Both effects, the high intermixing and WL reduction cause a drastic change in electronic levels, with the CL making up of 1–2 monolayers being the most effective configuration to reduce the radiative-recombination and minimize the potential barriers for carrier transport.

Defect influence on in-plane photocurrent of InAs/InGaAs quantum dot array: long-term electron trapping and Coulomb screening

Metamorphic InAs/In_0.15Ga_0.85As and InAs/In_0.31Ga_0.69As quantum dot (QD) arrays are known to be photosensitive in the telecommunication ranges at 1.3 and 1.55 μm, respectively; however, for photonic applications of these nanostructures, the effect of levels related to defects still needs in-depth investigation. We have focused on the influence of electron traps of defects on photocurrent (PC) in the plane of the QD array, studying by PC and deep level thermally stimulated current spectroscopy together with HRTEM and theoretical modeling. In the structures, a rich spectrum of electron trap levels of point defects EL6 (E _c - 0.37 eV), EL7 (0.29-0.30 eV), EL8 (0.27 eV), EL9/M2 (0.22-0.23 eV), EL10/M1 (0.16 eV), M0 (∼0.11 eV) and three extended defects ED1/EL3 (0.52-0.54), ED2/EL4 (0.47-0.48 eV), ED3/EL5 (0.42-0.43 eV) has been identified. Among them, new defect levels undiscovered earlier in InAs/InGaAs nanostructures has been detected, in particular, EL8 and M0. The found electron traps are shown to affect a time-dependent PC at low temperatures. Besides a long-term kinetics due to trap charging, a prolonged PC decrement versus time is measured under constant illumination. The decrement is interpreted to be related to a Coulomb screening of the conductivity channel by the electrons captured in the QD interface traps. The decrement is well fitted by allometric exponents, which means many types of traps involved in electron capturing. This study provides new findings into the mechanism of in-plane PC of QD arrays, showing a crucial importance of growth-related defects on photoresponsivity at low temperatures.