Dependence of the microstructural and the optical properties on the GaAs spacer thickness in InAs/GaAs double quantum dots grown by using the Indium-flush procedure

Abstract

Potential applications of quantum dots (QDs) have driven extensive efforts to grow high-quality QDs on semiconductor substrates by using various techniques [1]. Optoelectronic quantum devices utilizing QDs can be fabricated because QDs have discrete artificial atomic energy levels [2]. Thus, the microstructural and the optical properties of self-assembled QD systems have been particularly attractive for many years because of the potential application of QDs to optoelectronic devices [3–8]. In particular, the optical properties of QDs have been widely investigated because of interests in both fundamental physical properties and promising applications, such as QD lasers [9, 10], QD infrared photodetectors [11, 12], and QD memory devices [13] operating at lower current and at higher temperature. Among the many quantum dot structures, coupled quantum dot structures consisting of two smaller band-gap wells, which are different in band-gap energy and are separated by a thin embedded barrier, are currently receiving considerable attention for promising applications in electronic and optoelectronic devices [14–18]. Since the microstructural and the optical properties of coupled QDs are very important for optoelectronic devices based on QD structures, studies concerning the physical properties are still necessary in order to fabricate high-efficiency coupled QD devices. Many works on the formation and the physical properties of InAs/GaAs QDs grown by using the Stranski–Krastanov (S–K) growth mode have been performed. Even though some works on the sizes and the shapes of vertically stacked self-assembled QDs grown by using an indium-flush procedure have been performed [19–21], studies concerning the dependence of the microstructural and the optical properties on the GaAs spacer thickness in InAs/GaAs closely coupled double QDs grown by using the indium-flush method have not yet been reported. While the size of the upper QD in a coupled double QD grown by using the S–K mode is larger than that of the lower QD due to a strain field [22], the sizes of the two QDs in the coupled QDs grown by using the S–K mode, together with an indium-flush procedure, are almost the same. Since the top and the bottom dots of a double QD having similar sizes and shapes is important for investigating the coupling effects of double QDs and their promising applications in electronic and optoelectronic devices, the indium-flush method, which is an effective way to control precisely the sizes and the thicknesses of the QDs, should be very useful for investigating the coupling behavior in double QDs [19]. Furthermore, very few works on the dependence of the activation energy of double QDs on the spacer thickness have been done. This letter reports the dependences of the microstructural and the optical properties on the GaAs spacer thickness in InAs/GaAs double QDs with different GaAs spacer layers grown by using molecular beam epitaxy (MBE) with an indium-flush method. Transmission electron microscopy (TEM) measurements were performed to characterize the microstructural properties of the InAs/GaAs double QDs, and photoluminescence (PL) measurements were carried out in order to investigate the dependences of the full width at half maximum (FWHM), the peak position of the interband transitions, and the activation energy on the I. Y. Jung AE Y. M. Park AE Y. J. Park AE J. I. Lee Nano-Device Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea