Nominal PbSe nano-islands on PbTe: grown by MBE, analyzed by AFM and TEM
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Nominal PbSe nano-islands on PbTe: grown by MBE, analyzed by AFM and TEM Peter Moeck1, Mukes Kapilashrami1*, Arvind Rao1*, Kirill Aldushin2, Jeahuck Lee3, James E. Morris3, Nigel D. Browning4, and Patrick J. McCann5 1
Portland State University (PSU), Department of Physics, P.O. Box 751, Portland, OR 97207-0751, USA ([email protected]), * Department of Materials Science, The Royal Institute of Technology, Brinellvägen 23, S-100 44 Stockholm, Sweden 2 Institut für Geologie, Mineralogie und Geophysik der Universität Bochum, Universitätsstraße 150, D- 44780 Bochum, Federal Republic of Germany 3 Portland State University, Department of Electrical & Computer Engineering, P.O. Box 751, Portland, OR 97207-0751, USA 4 Department of Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, CA 95616; and National Center for Electron Microscopy, MS 72-150, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 5 University of Oklahoma, School of Electrical and Computer Engineering, Norman, OK 73019, USA ABSTRACT Nominal PbSe nano-islands were grown in the Stranski-Krastanow mode on (111) oriented PbTe/BaF2 pseudo-substrates by molecular beam epitaxy (MBE). The number density and morphology of these islands were assessed by means of atomic force microscopy (AFM). Transmission electron microscopy (TEM) was employed to determine the strain state and crystallographic structure of these islands. On the basis of both AFM and TEM analyses, we distinguish between different groups of tensibly strained islands. The suggestion is made to use such nano-islands as part of nanometrology standards for scanning probe microscopy. INTRODUCTION Heteroepitaxial semiconductor quantum dots have over the last decade resulted in “paradigm changes in semiconductor physics and technology” [1]. Improved optoelectronic devices (such as lasers and mid-infrared detectors) and novel nanoelectronics concepts which depend on single electron transport and tunneling have been demonstrated [1-5]. The PbSe/PbTe quantum dot system [6-9] is particularly interesting for mid-infrared vertical cavity surface emitting lasers [10]. Unlike most III-V and II-VI compound semiconductor quantum dot systems, strong confinement of both holes and electrons can readily be achieved in lead salt quantum dots. The Bohr radii of the holes and electrons are of the same order of magnitude, resulting for PbSe in an exciton Bohr radius on the order of 50 nm. Materials science aspects of heteroepitaxial quantum dot growth are extensively discussed in the above mentioned reviews [1-5], but atomic ordering and phase separation in such heteroepitaxial structures are a rather new field of study [11-13]. This is surprising, as it has been known since 1985 that atomic ordering and phase separation exists in basically all technically important heteroepitaxial column IV element, III-V, and II-VI compound semiconductor alloy structures that possess in the disordered state the diamond or sphalerite (zinc blende) structure [14,15]. Atomic
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