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Phys. Rev. Lett. 112, 155502 – Published 18 April 2014.
K. J. Rietwyk 1,*, Y. Smets1, M. Bashouti2,3, S. H. Christiansen3, A. Schenk1, A. Tadich4, M. T. Edmonds5, J. Ristein6, L. Ley1,6, C. I. Pakes1,†
1Department of Physics, La Trobe University, Victoria 3086, Australia and
2Max-Planck-Institute for the Science of Light, D-91058 Erlangen, Germany and
3Institute of Nanoarchitectures for solar energy conversion, Helmholtz-Centre Berlin (HZB), D-14109 Berlin, Germany and
4Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia and
5School of Physics, Monash University, Clayton, Victoria 3800, Australia and
6Technische Physik, Universität Erlangen, D-91058 Erlangen, Germany.
Abstract
We demonstrate a novel doping mechanism of silicon, namely n-type transfer doping by adsorbed organic cobaltocene (CoCp2*) molecules. The amount of transferred charge as a function of coverage is monitored by following the ensuing band bending via surface sensitive core-level photoelectron spectroscopy. The concomitant loss of electrons in the CoCp2* adlayer is quantified by the relative intensities of chemically shifted Co2p components in core-level photoelectron spectroscopy which correspond to charged and neutral molecules. Using a previously developed model for transfer doping, the evolution in relative intensities of the two components as a function of coverage has been reproduced successfully. A single, molecule-specific parameter, the negative donor energy of −(0.50±0.15) eVsuffices to describe the self-limiting doping process with a maximum areal density of transferred electrons of 2×1013 cm−2 in agreement with the measured downward band bending. The advantage of this doping mechanism over conventional doping for nanostructures is addressed.
Additional Information
The doping of semiconductors has been the cornerstone of modern electronics for the past 50 years and during this time has relied on the incorporation of donor and acceptor atoms into the host semiconductor. While this has proven to be exceedingly effective for bulk materials at micro-scales, there are numerous outstanding challenges that impede doping of semiconductor materials in various device architectures at he true nano-scale. At these sizes non-negligible spatial variation in doping concentration will likewise affect carrier concentration and ultimately device performance. In the case of silicon nanowires, an emerging and promising technology, dielectric mismatch between the core and surroundings of silicon nanowires leads to poor screening of the dopant Coulomb potential. The result is inefficient doping and increasing resistance with reduced nanowire diameter. Naturally we sought an alternate doping mechanism to circumvent these limitations.
In this paper we demonstrate a novel mechanism for doping silicon, namely n-type transfer doping by adsorbed organic cobaltocene (CoCp2*) molecules. The amount of transferred charge as a function of coverage is monitored by following the ensuing band bending via surface sensitive core-level photoelectron spectroscopy (XPS). The concomitant loss of electrons in the CoCp2* adlayer is quantified by the relative intensities of chemically shifted Co2p components in XPS which correspond to charged and neutral molecules. Using a previously developed model for transfer doping, the evolution in relative intensities of the two components as a function of coverage has been reproduced successfully. A single, molecule-specific parameter, the negative donor energy of –(0.50±0.15) eV suffices to describe the self-limiting doping process with a maximum areal density of transferred electrons of 2 × 1013 cm-2 in agreement with the measured downward band bending.
The transfer doping mechanism offers a number of advantages over conventional doping. The former process is thermodynamically favourable and no annealing is required to activation the doping mechanism. The observed negative donor energy enables effective doping at the lowest temperatures without carrier freeze-out. Furthermore, the spatial separation of dopant atoms from the conductive channel is likely to improve carrier mobility by reducing impurity scattering. The observed current densities are comparable to those achieved in conventional silicon transistors, highlighting the feasibility of transfer doping for applications.
