Evolution of wetting layer in InAs/GaAs quantum dot system

evolution of the wetting layer (WL) with the InAs deposition thickness has been studied by reflectance difference spectroscopy (RDS). Two transitions related to the heavy-and light-hole in the WL have been distinguished in RD spectra. Taking into account the strain and segregation effects, a model has been presented to deduce the InAs amount in the WL and the segregation coefficient of the indium atoms from the transition energies of heavy-and light-holes. The variation of the InAs amount in the WL and the segregation coefficient are found to rely closely on the growth modes. In addition, the huge dots also exhibits a strong effect on the evolution of the WL. The observed linear dependence of In segregation coefficient upon the InAs amount in the WL demonstrates that the segregation is enhanced by the strain in the WL. A strained epilayer with a thickness beyond a critical value will change from layer-by-layer growth (Frank– van der Merwe growth) to island growth (Stranski– Krastanow growth) in order to relax strain energy. When covered by barrier materials, these self-assembled nanometer-scale islands become the so-called quantum dots (QDs), which have attracted much attention due to the interest in their fundamental physics and potential application in novel devices [1]. The optical and electrical properties of the self-assembled QDs can be affected greatly by the wetting layer (WL) around the dots [2–5]. The WLs serve as channels for carriers to fall into dots and to redistribute between the dots, strongly influencing on the emission properties of the dots [2, 3]. It was also predicted that the WLs could limit greatly the modulation response of QD lasers [4]. Therefore the information of electronic states and structures of WLs is desirable for understanding the properties of the QDs and their devices. However, unlike QDs, which can be characterized straightly by atomic force microscopy (AFM), transmission electron microscopy (TEM) and photolumi-nescence (PL), etc. [1], it is quite difficult to access experimentally the information of WLs. Occasionally WLs can be observed in PL excitation and photore-flectance (PR) measurements [6]. The direct observation has been achieved by the cross-sectional STM measurements under an ultra-high vacuum environment [7, 8]. However, there is still no systematical experiment research on the evolution of WLs up to now. In this letter, reflectance difference spectroscopy (RDS) will be adopted to characterize the WL in the InAs/GaAs system. RDS is a sensitive tool for characterizing the in-plane optical …