Silicide formation during Mn doping of Ge/Si (001) self-assembled quantum dots
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Matthieu Jamet Institut Nanosciences et Cryogénie/SP2M, CEA-Université Joseph Fourier, F-38054 Grenoble, France
Petra Reinke and Jerrold A. Floroa) Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904 (Received 27 August 2013; accepted 29 October 2013)
Heteroepitaxial Ge quantum dots were grown on Si (001) by molecular beam epitaxy, with a Mn co-deposition flux giving a nominal composition of Ge0.9Mn0.1. At this large Mn flux, and with growth temperatures of 450 °C required for Ge quantum dot self-assembly, extensive second phase formation occurred. Atomic force microscopy reveals that quantum dots typical for the Ge/Si (001) system still form. In addition, copious formation of both rod-like and cluster-like morphologies is observed, with many of these structures conjoined to Ge dots. Extensive transmission electron microscopy identified several coexisting intermetallic phases, all based on Mn–silicide crystal structures, albeit with varying degrees of Ge substitution. The Ge quantum dots themselves appear to have little or no Mn incorporated in them, indicating that the intermetallic particles scavenge Mn from extended surface areas. Under these growth conditions, Mn is highly mobile, with surface diffusion lengths of the order of 800 nm, with significant bulk mobility as well, resulting in surface structures that also penetrate the Si substrate. A magnetic phase transition at 220 °C does not match known behavior of the bulk silicide phases but might result from extensive ternary alloying with Ge, especially into the cubic MnSi phase.
I. INTRODUCTION
Semiconductors can be functionalized via transition metal doping to form the dilute magnetic semiconducting (DMS) phase,1 with Mn serving as a common magnetic dopant in a number of semiconducting hosts. The technological attraction of the DMS phase is that it could afford electric-field control over the magnetic response, if coupling between Mn ions occurs via holes whose concentration can be gate-modulated. Extensive work has been done on group IV DMS materials since evidence for carrier-mediated ferromagnetic order in epitaxial Mn-doped Ge/Ge (001) was first reported in 2002 by Park et al.,2 who demonstrated Tc increasing linearly with the Mn concentration from 25 to 116 K. Since then, many other groups have investigated the synthesis of Mn doped-Ge using a variety of techniques. The most thorough examination has been for homoepitaxial growth of Mn-doped Ge films on Ge (001) substrates.3–11 Given the nearly complete lack of solubility of Mn in Ge,12 formation of precipitate-free MnxGe1x DMS requires low growth temperatures, although reports differ on the upper bound temperature due to differences in other experimental a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.346 3210
J. Mater. Res., Vol. 28, No. 23, Dec 14, 2013
http://journals.cambridge.org
Downloaded: 14 Mar 2015
conditions, such as deposition rate. However, 180 °C seems to be a rough upper bound on growth
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