In situ HREM Study on the Thermal Stability of Atomic Layer Epitaxy Grown InAs/GaAs Quantum Dots
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In situ HREM Study on the Thermal Stability of Atomic Layer Epitaxy Grown InAs/GaAs Quantum Dots H. S. Kim1, J. H. Suh1, C. G. Park1, S. J. Lee2, S. K. Noh2, J. D. Song3, Y. J. Park3, W. J. Choi3, and J. I. Lee3 1 Department of Materials Science & Engineering Pohang University of Science and Technology (POSTECH), Pohang, Korea 2 Quantum Dot Technology Laboratory, Korea Research Institute of Standards and Science, (KRISS), Daejeon, Korea 3 Nano Device Research Center, Korea Institute of Science & Technology (KIST), Seoul, Korea ABSTRACT Self-assembled InAs/GaAs quantum dots (QDs) were grown by the atomic layer epitaxy technique and the structure and the thermal stability of QDs have been studied by using high resolution electron microscopy with in-situ heating experiment capability. The QDs were found to form a hemispherical structure with {136} side facet in the early stage of growth. The average height and diameter of the QD were found to be ~ 5.5 nm and ~ 23 nm, respectively. Upon capping by GaAs layer, however, the apex structure of QD changed to a flat one. In-situ heating experiment within TEM revealed that the uncapped QD remained stable until 580oC. However, at temperature above 600oC, the QD structure became flat due to the fast decrease of QD height. After flattening, the atoms diffused from the InAs QD to the GaAs substrate, resulting in the total collapse. The density of the QD decreased abruptly by this collapse and most QDs disappeared at above 600oC. INTRODUCTION Compound semiconductors such as GaAs, InAs and InP have superior optoelectrical properties because of the high mobility of carriers and the direct transition of the energy band gap. Compound semiconductor heterostructures, which have different energy band gap, have recently been grown easily owing to the technical development of epitaxial thin film growth. Therefore, extensive studies have been carried out on the heterostructures of compound semiconductors, for example quantum wells [1], quantum wires [2] and quantum dots [3]. Quantum dot (QD) devices have high current gain, low threshold voltage and thermal stability because the QD has an atomic-like electron density of states [4-5]. Thus, considerable effort has been devoted to fabricate lasers [6], advanced memories [7] and infrared photodetectors [8] by using selfassembled QDs. For high quality QD growth, process parameter effects such as growth temperature, growth rate, InAs thickness, etc. on QD growth characteristics should be understood
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quantitatively. In particular, growth temperature is important for the control of QD structure because the diffusion of In and As atoms on the substrate depends significantly on temperature. Since the surface diffusion of In and As atoms plays a critical role in determining the QD structure, understanding the high temperature behavior and thermal stability of the QD structure is important. In the present study, InAs QD was grown on GaAs substrate by means of atomic layer epitaxy (ALE) deposition and the structure and the thermal stab
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