Role of Sb on the vertical-alignment of type-II strain-coupled InAs/GaAsSb multi quantum dots structures

Highlights

• Phenomena in high-density MQD InAs/GaAsSb with fine GaAs spacers.

• GaAsSb capping allow QDs coupling.

• QDs coupling affect the Sb distribution.

• Strong Sb accumulation on QDs apex, forming a collar.

• The Sb inhibit the formation of agglomerations.

Abstract

The implementation of GaAs_0.8Sb_0.2 as CL to obtain type-II strain-coupled InAs MQD structures has been examined and compared to similar structures without Sb or without strain coupling. First, it has been demonstrated that capping with GaAsSb prevents the formation of In-rich agglomerations that hampered the QD formation as it has been observed in the sample without Sb. Instead, it promotes the vertical alignment (VA) of almost all QDs with a high density of QD columns. Second, there is a preferential Sb accumulation over the dots together with an undulation of the growth front, contrary to the observed in the uncoupled structure. In case of a deficient covering of GaAsSb, as occurs for giant QDs, In-rich agglomerations may develop. Each VAQD column consists of a sequence of alternating quantum blocks of pyramid-shaped In(Ga)As separated by GaAsSb blocks that rest over them. These Sb-rich blocks are not homogeneous accumulating around the pyramidal apex like a collar. Between the columns, there is an impoverishment of In and Sb compared to the uncoupled sample. These columns can behave as self-aligned nanowires with type II band alignment between self-assembled InAs and GaAsSb quantum blocks that opens new opportunities for novel devices.

Introduction

Self-assembled heteroepitaxial InAs quantum dots (QDs) grown via Stranski-Krastanov mode keep attracting special attention since they are able to enhance the properties of many optoelectronic devices [1,2]. In this field, augmenting QD density is a potential way of increase the efficiency in these devices and the so called vertically aligned (VA) structures of InAs/GaAs QDs have been broadly investigated, which consist of the closely stacking of several InAs QD layers [3,4]. The use of VAQDs results in larger QDs with improved size uniformity and with an attractive electronic coupling that modifies their optical properties [5,6]. Certainly, longer wavelength emissions could be achieved by growing both larger and strain-coupled QD heterostructures [7,8].

In this sense, the use of GaAsSb capping layers (CL) on InAs QDs for Sb contents above 16% has attracted special attention because of the band alignment becomes type II, with the holes localized outside the QD (in the CL) increasing the radiative carrier lifetime [[9], [10], [11], [12]]. These kind of type II QDs are recommended in quest of an increased sub-bandgap current with less voltage reduction to envisage interband solar cells (IBSC) that should exceed the efficiency of standard QD solar cells [13]. In addition, the introduction of Sb into the CL may bring extra benefits due to its surfactant effect that reduces the surface free energy, by suppressing the coalescence of neighbouring QDs, improving uniformity and reducing non-radiative centres [14,15]. However, although the behaviour of GaAsSb CLs in uncoupled InAs QD layers has been extensively studied, there are very few works about combining the growth of InAs VAQD structures using GaAsSb CLs with type-II alignment. Kim et al., [16] analysing different thicknesses of GaAs_0.83Sb_0.17 spacers embedding in a ten-stack InAs QD structure, showed a great luminescence properties and higher carrier thermal stability for the 10-nm thickness. Liu et al., analysing several 10-layer VAQDs with different GaAs/GaAsSb spacers (x < 0.2), found a type-II behaviour by means of time-resolved photoluminescence (TRPL) and power dependence (PD) PL measurements when GaAs_0.8Sb_0.2 spacers are used [17]. However, the structural and compositional characteristics of this sample are not presented, probably because of its worse structural characteristics that would explain its poor results in external quantum efficiency (EQE) and current-voltage measurements at one-sun illumination [18]. Very recently Panda et al. [19,20], have investigated the impact on the band alignment and carrier lifetime in strain-coupled InAs QDs capped with GaAsSb/GaAs CLs using different Sb-content and thickness but only for the case of bilayer structures.

The aim of this work is to study the behaviour of Sb in type-II strain-coupled InAs/GaAsSb/GaAs MQD structure in comparison with both (i) its equivalent structure without Sb and (ii) without strain coupling by inserting wide GaAs spacers. As can be seen in the article, the use of GaAsSb CLs in strain-coupled structures changes not only the size, shape and density of the QDs but also the way the Sb is distributed within the structure.