(S)TEM Analysis of the Strain and Morphology of InAs Quantum Dots using GaAs(Sb)(N) Capping Layers for Solar Cell Applications

Abstract

transki-Krastanov In(Ga)As/GaAs quantum dots (QDs) are the subject of great scientific and technological interest, primarily aimed to broaden the emission wavelength towards 1.3-1.55 μm, required for optical fiber telecommunications [1]. More recently, this QD system has drawn a lot of attention owing to their unique properties, which could be also used to boost the energy conversion efficiency of solar cells and exceed the Schockley-Queisser limit [2]. Additionally, the utilization of capping layers (CLs) with materials different than GaAs covering the InAs QDs may provide all these approaches with extra advantages. In this respect, strain-reducing layers (SRLs) such as GaAsSb extend the emission wavelength by reducing the valence band offset while strain compensated layers (SCLs) such as GaAsN CL improve the InAs QD luminescence extending the emission wavelength via conduction band offset reduction, together with a compensation of the compressive strain derived from the InAs QDs. As a consequence, the use of GaAsSbN quaternary alloys as a CL for InAs/GaAs QDs could open even more possibilities allowing independent tailoring of the electron and hole confinement potentials and the strain states of the QDs in a wide range, which could be useful for many applications [3]. However, the nature and the growth conditions of such CLs are critical parameters to determine the resulting properties of the QDs. Processes such as intermixing, segregation or strain-enhanced diffusion could occur during the capping process that modify the shape, composition and strain fields of the QDs [4]. The aim of this study is to compare in a statistical way the effect of several capping materials with different designs and growth conditions of the CL on the morphology of InAs/GaAs QDs.