IEICE TRANS. ELECTRON., VOL.E95–C, NO.2 FEBRUARY 2012 247 BRIEF PAPER Special Section on Photonic Devices using Nanofabrication Technology and Their Applications Broadband Light Source Based on Four-Color Self-Assembled InAs Quantum Dot Ensembles Monolithically Grown in Selective Areas Nobuhiko OZAKI †a) , Koichi TAKEUCHI † , Shunsuke OHKOUCHI †† , Naoki IKEDA ††† , Nonmembers, Yoshimasa SUGIMOTO ††† , Kiyoshi ASAKAWA †††† , Members, and Richard A. HOGG ††††† , Nonmember SUMMARY We developed advanced techniques for the growth of self- assembled quantum dots (QDs) for fabricating a broadband light source that can be applied to optical coherence tomography (OCT). Four QD en- sembles and strain reducing layers (SRLs) were grown in selective areas on a wafer by the use of a 90 ◦ rotational metal mask. The SRL thickness was varied to achieve appropriate shifts in the peak wavelength of the QD emission spectrum of up to 120 nm. The four-color QD ensembles were expected to have a broad bandwidth of more than 160 nm due to the com- bination of excited state emissions when introduced in a current-induced broadband light source such as a superluminescent diode (SLD). Further- more, a desired shape of the SLD spectrum can be obtained by controlling the injection current applied to each QD ensemble. The broadband and spectrum shape controlled light source is promising for high-resolution and low-noise OCT systems. key words: quantum dot, selective-area growth, metal mask, optical co- herence tomography, superluminescent diode 1. Introduction Self-assembled quantum dots (QDs) have come under in- tense study as a promising nano-material for the devel- opment of a variety of electronic and optoelectronic de- vices. Some studies have examined In(Ga)As-QD-based laser diodes and the broadband light source in the commer- cial base [1]–[3]. Self-assembled In(Ga)As-QDs are epi- taxially grown via the Stranski-Krastanov (S-K) mode [4], which is driven by strains due to some lattice mismatch be- tween the epitaxial layer and the substrate. In general, QDs grown in the S-K mode are randomly distributed in position and size over the whole substrate. However, for advanced applications of self-assembled QDs to electronic and optoelectronic integrated devices, selective-area growth (SAG) of QDs is a key technology. For instance, our proposed integrated all-optical device based on photonic crystal waveguides including QD ensem- bles with different absorption wavelengths requires SAG of Manuscript received June 22, 2011. Manuscript revised October 28, 2011. † The authors are with the Wakayama University, Wakayama- shi, 640-8510 Japan. †† The author is with the NEC Corporation, Tsukuba-shi, 305- 8501 Japan. ††† The author is with the NIMS, Tsukuba-shi, 305-0047 Japan. †††† The author is with the University of Tsukuba, Tsukuba-shi, 305-8571 Japan. ††††† The author is with the University of Sheffield, North Campus, Broad Lane, Sheffield, S3 7HQ, UK. a) E-mail: ozaki@sys.wakayama-u.ac.jp DOI: 10.1587/transele.E95.C.247 QD ensembles on a wafer at a scale of several tens of mi- crometers [5]. In the previous work, we developed a method for the required growth of QDs in selective areas with two different emission/absorption wavelengths by using a 180 ◦ rotational metal mask (MM) combined with conventional molecular beam epitaxy (MBE) [6]. Based on the SAG method, we recently proposed a broadband light source based on four-color QD ensembles, as shown in Fig. 1(a) [7]. This can be applied to opti- cal image system using a low-coherence light source, so- called the optical coherence tomography (OCT) system de- picted in Fig. 1(b). The OCT system, which is based on low-coherence interferometry, consists of a low-coherence Fig. 1 (a) Spectrum-shape-controlled broadband light source based on four-color QD ensembles. (b) Schematic image of an OCT system. Copyright c  2012 The Institute of Electronics, Information and Communication Engineers