Surfactant-mediated growth of InAs– GaAs superlattices and quantum dot structures grown at different temperatures M. Alduraibi, C. Mitchell, S. Chakraborty, M. Missous à Microelectronics and Nanostructures Group, School of Electrical and Electronic Engineering, The University of Manchester, Sackville Street, Manchester, PO Box 88, M60 1QD, UK article info Available online 22 July 2008 Keywords: Molecular beam epitaxy Surfactant growth Quantum dots Low-temperature growth abstract The structural and optical qualities of superlattice InAs–GaAs structures and quantum dots (QDs), grown by molecular beam epitaxy (MBE) at low (250 1C) and normal (450 1C) growth temperatures, have been investigated. The InAs layers (3 monolayers) were grown under conditions where only the indium beam impinged upon the growth surface (surfactant growth mode). This growth mode still resulted in the formation of QDs at normal growth temperatures, but with dot sizes that were much smaller than those for ‘‘normal’’ growth of 3 ML InAs–GaAs QD structures. In addition, at low temperature under such ‘‘arsenic-free’’ conditions a very high quality InAs–GaAs superlattice structure with 3 ML of InAs was formed, as demonstrated by transmission electron microscopy (TEM). This is a direct confirmation that the critical thickness of InAs can be extended well beyond the 1.7 ML limit seen at higher growth temperatures. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Semiconductor quantum dots (QDs) have been extensively investigated in recent years due to their unique optical properties arising from their discrete energy levels, which make them useful for optoelectronic applications. InAs QDs have attracted much attention due to the effective bandgap energy, which allows emission and absorption at telecommunication wavelengths (1.3 and 1.55 mm) where silica fibres have minimum losses. In addition, low-temperature (LT)-grown InAs–GaAs superlattices are useful materials for ultrafast optical switches due to the short carrier lifetime caused by the incorporation of excess arsenic into the crystal as point defects [1,2]. Molecular beam epitaxy (MBE) is one of the key techniques used to fabricate self-organized QD structures, formed by the Stranski–Krastanow growth mode. It has been observed that the growth conditions have an important effect on the properties of self-organized QDs, such as their density and size distribution [3]. Several studies have been made on single-plane InAs QDs, grown on GaAs at different growth temperatures [4,5]. However, relatively little has been reported on stacked binary InAs QDs grown at normal growth temperature (450 1C) and InAs–GaAs superlattices grown at LT (250 1C). It has been found that, under normal growth conditions (NGC), a structure of 3 monolayers (MLs) of stacked binary InAs–GaAs QDs contained volcano-like defects [6]. The effect of such defects on the quality of the material structure was clearly observed in double crystal X-ray diffraction (DCXRD) measurements, which showed broad satellite peaks. The work presented here demon- strates that using only the indium beam (surfactant growth) during the growth of thick InAs layers (3 MLs) improves the structural quality of InAs–GaAs QDs and superlattices enor- mously. Detailed DCXRD, transmission electron microscopy (TEM), photoluminescence (PL) and atomic force microscopy (AFM) measurements are reported for samples grown using this technique at both low and high temperatures for the first time. 2. Experiments Two novel surfactant-grown structures were investigated in this study (identified as sample no. 1949 and no. 1753) alongside similar samples grown under NGC. All samples were grown using an Oxford Instruments VG V90H MBE reactor on semi-insulating (10 0) GaAs substrates. The growth of sample no. 1949 started with a 170nm buffer layer, grown at 580 1C. Then, the substrate temperature was decreased to 450 1C for the growth of ten periods of 25nm GaAs layer (spacer) followed by an InAs layer with a nominal thickness of 2.9 MLs. The sample was left uncapped for the purpose of AFM imaging. The growth of the InAs layers was performed with just the indium beam impinging on the surface, surfactant growth; under these conditions the group V uptake was provided by the residual arsenic in the growth chamber. The growth rate of GaAs and InAs was 1 and 0.033 ML s 1 , respectively. The second sample (sample no. 1753) has an identical structure ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/mejo Microelectronics Journal 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.06.055 à Corresponding author. Tel.: +44161 200 4797; fax: +44161 200 4669. E-mail address: m.missous@manchester.ac.uk (M. Missous). Microelectronics Journal 40 (2009) 476– 478