Self-assembled Si/Ge quantum dot structures for novel device applications
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Self-assembled Si/Ge quantum dot structures for novel device applications K. Brunner, D. Bougeard, A. Janotta, M. Herbst, P. H. Tan, H. Riedl, M. Stutzmann, and G. Abstreiter Walter Schottky Institute, TU Munich, Am Coulombwall, D-85748 Garching, Germany ABSTRACT The band structure of self-assembled Si/Ge quantum dot structures deposited by molecular beam epitaxy in the Stranski Krastanov growth mode is characterized by optical and electrical spectroscopy. Interband and intraband absorption, photocurrent, photoluminescence, Raman and admittance spectroscopy of structures with quantum dots of about 20 nm lateral size offer insight into the discrete level scheme within the valence band, the optical transitions and the lifetime of localized hole states. The results are discussed with respect to their possible applications in infrared light detection, storage and quantum-logic devices.
INTRODUCTION The heteroepitaxial deposition of strained material in the Stranski-Krastanow (S-K) growth mode is a very powerful technique applied for synthesis of a huge number of highquality semiconductor quantum dots [1]. The quantum dot properties like size, composition, areal density and layer separation can be designed by the growth parameters. Especially for self-assembled Ge islands on (001)Si substrate, the physical island properties can be tuned in a wide range. The lateral size (height) of islands, for example, spans from about 10 –20 nm (2 nm) observed for substrate temperatures of about TS= 500°C to about 200 nm (12 nm) observed at about TS=750°C. Large Ge islands embedded in a Si matrix without defects may offer practical advantages like increased Ge contents and increased effective band offsets compared to strained quantum well layer structures that suffer from limitations due to misfit dislocation formation. They enlarge the spectral range, for example, of interband as well as intraband Si/Ge photodetectors. Here we focus on studies of the small type of islands that are called “Ge hut clusters” due to their facetted shape observed by atomic force microscopy prior to overgrowth by Si [2]. They represent quantum dots (QD), in which hole states are localized in quasi 0-dimensional discrete levels. Direct band gap quantum dots like GaAs/InAs QDs have been extensively studied by optical spectroscopy of single quantum dots and a deep understanding of band structure, many-particle effects, and optical properties of excitonic 0D systems was developed [3]. Si/Ge quantum dot structures reveal indirect optical interband transitions of discrete hole states localized within Ge dots and electron states in the surrounding Si host that are just weakly bound by Coulomb attraction of holes and by local strain fields [1, 4]. Single quantum dot spectroscopy is not reported so far from Si/Ge, due to the low PL intensity. In the next section, we describe the results of optical interband and intra valence band spectroscopy and admittance studies of Si/Ge QD ensembles. In conclusion, we discuss their possible impact on novel devices based
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