Ground-state energy trends in single and multilayered coupled InAs/GaAs quantum dots capped with InGaAs layers: Effects of InGaAs layer thickness and annealing temperature S. Shah b , K. Ghosh a , S. Jejurikar a , A. Mishra a , S. Chakrabarti a, * a Center for Excellence in Nanoelectronics, Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India b Energy Science and Engineering Department, Indian Institute of Technology Bombay, Mumbai 400076, India 1. Introduction In recent years, semiconductor quantum dots (QDs) have been of great research interest from both experimental and theoretical points of view. Researchers involved in the fabrication of optical devices, such as lasers and telecommunication devices in the 1.3– 1.55 mm range, have been able to achieve long distance transmis- sion without amplification; thus, yielding high efficiency at a low cost. Several investigations have predicted that devices fabricated using QDs can have high gain, low threshold current, and a high characteristic temperature [1–3]. Research has demonstrated the use of InAs/GaAs QDs in the field of mid-wave and long-wave infrared emissions [4], but further research into the growth processes and optical properties of InAs QDs on GaAs surfaces is needed. Researchers have used vertical coupling of stacked layers of InAs QDs separated by GaAs layers to induce uniformity in the size distribution of the QDs. Such heterostructures exhibit a greater red-shift in electronic emission levels than those consisting of single-layer QDs [5]. Stacked QD structures have been extensively used to increase the modal gain of QD lasers; however, accumulation of strain, due to a lattice mismatch between the surface material and the deposited material, can cause dislocations that greatly degrade the performance of such lasers. For such applications, it is necessary to have low strain in their stacked QD structures. To extend emission wavelengths, several studies have used strain-reducing layers (SRLs), such as those comprised of InGaAs and InAlAs, with different lattice constants as a way to modify QD structure [6–8]. Covering QDs with SRLs reduces the QDs’ emission energy by lowering vertical stain, increasing QD size, and increasing strain-driven decomposition of the InGaAs layers [9]. Hence, our interest in comparing single-layer QD structures with vertically coupled multilayered QD structures. We have investi- gated the effects of SRLs on the structure and emission energy levels of QDs. Here, we report results of our temperature- dependent photoluminescence (PL) investigations of single and multilayer QD structures grown using molecular beam epitaxy (MBE) but with variation in InGaAs layer thickness. 2. Experimental Single-layer and vertically coupled multilayered InAs/GaAs heterostructures were grown in an EPI MOD GEN II MBE system. The schematic of the four structures in this study are shown in Fig. 1. Single-layer (2.7 monolayers, ML) InAs QD heterostructures were grown at 520 8C at a fixed growth rate of 0.187 ML/s. The single QDs had a capping layer of InGaAs with thicknesses of 5 nm Materials Research Bulletin 48 (2013) 2933–2939 A R T I C L E I N F O Article history: Received 23 May 2012 Received in revised form 4 April 2013 Accepted 16 April 2013 Available online 22 April 2013 Keywords: A. Nanostructures A. Semiconductors B. Crystal growth A B S T R A C T Vertically coupled, multilayered InAs/GaAs quantum dots (QDs) covered with thin InGaAs strain- reducing layers (SRLs) are in demand for various technological applications. We investigated low temperature photoluminescence of single and multilayered structures in which the SRL thickness was varied. The SRL layer was responsible for high activation energies. Deviation of experimental data from the Varshni (1967) model, E(T) = E  1 T 2 /T + b, suggests that the InAs-layered QDs have properties different from those in bulk material. Anomalous ground-state peak linewidths (FWHM), especially for annealed multilayer structures, were observed. A ground-state peak blue-shift with a broadened linewidth was also observed. Loss of intensity was detected in samples annealed at 800 8C. Presence of SRLs prevents formation of non-radiative centers under high temperature annealing. The results indicate the potential importance of such structures in optoelectronic applications. ß 2013 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +91 22 2576 7421. E-mail addresses: subho@ee.iitb.ac.in, subhanandachakrabarti@gmail.com (S. Chakrabarti). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.04.028