Time-Integrated Photoluminescence Studies of In0.3Ga0.7As/GaAs Quantum Dot Molecules
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Time-Integrated Photoluminescence Studies of In0.3Ga0.7As/GaAs Quantum Dot Molecules William Kerr, Valeria Gabriela Stoleru, and Anup Pancholi Materials Science and Engineering, University of Delaware, 201 Dupont Hall, Newark, DE, 19716 ABSTRACT We investigate experimentally and theoretically optical and electronic properties of In0.3Ga0.7As/GaAs quantum dot molecules (QDMs) formed by two layers of self-assembled, vertically stacked quantum dots (QDs). Structures with In0.3Ga0.7As/GaAs QD layers separated by a thin GaAs barrier were grown by solid source molecular beam epitaxy, and were characterized by time-integrated photoluminescence (PL). For the temperature-dependent PL measurements a He-flow optical cryostat was used to control the temperature between 4 and 300 K. The experimentally observed behavior is in good agreement with that expected from our eight-band k·p calculations. Optical and electronic properties of these QDMs are further compared with those of dots grown under conditions that did not promote vertical organization. INTRODUCTION Three-dimensionally confined semiconductor quantum structures have been identified as potential candidates for various device applications such as lasers [1], detectors [2], single electron transistors [3], and quantum computers [4]. The ability to tailor the electronic structure of these structures has made them very interesting for basic as well as applied research. Quantum dot molecules defined by pairs of vertically stacked, electronically coupled QDs have been proposed as suitable structures for various applications in the Terahertz (THz) spectral regime. It has been predicted theoretically [5-7] that the QDM-based THz sources will be more effective than those based on quantum wells (QWs). The essential advance of the QDM-based lasers is the increased electron lifetime on excited levels due to the phonon bottleneck effect. This condition should facilitate the appearance of the intraband population inversion. The demonstration of QDM-based THz radiation sources will be of great importance due to the present lack of convenient, coherent, and compact solid-state sources. The utilization of selfassembled QD nanostructures allows for engineering the intraband transitions [8] and leads to reduced absorption losses introduced by free-carrier, phonon, and Auger processes, because of the reduced dimensionality in momentum space. In this article we report time integrated, temperature and excitation dependent PL studies of In0.3Ga0.7As/GaAs QDMs grown by solid source molecular beam epitaxy. Optical and electronic properties of the QDMs are compared with those of vertically isolated, decoupled dots (QDDs), grown under conditions that did not promote vertical self-organization. These studies provide valuable information that is directly useful in achieving a better understanding of the physical effects involved in optical transitions in QDMs, and in the development of new optical devices. EXPERIMENTAL DETAILS The structures were grown by solid source molecular beam ep
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