NSTI Nanotech 2009

Towards optimal spin injection and detection contacts on silicon/germanium

S. Ganguly, W. Van Roy, R. Lieten, R. Jansen, S. Kaushal, K. Sugishima
University of Texas at Austin, US

Keywords: spintronics, spin injection, contact resistance, silicon, germanium


We have fabricated and characterized ferromagnet-insulator-semiconductor (FMIS) contacts for electrical spin injection and detection with silicon-compatible technology. These contacts need their resistances to be carefully tuned within a narrow window with respect to the channel resistance in order to achieve high magnetoresistance between ferromagnetic source and drain. FMIS contacts on Si and GaAs have generally been too resistive: either due to Fermi-level pinning, or due to metal-semiconductor work-function mismatch for de-pinned contacts. We outline two approaches for achieving the optimally resistive spin contacts on silicon and germanium. We first apply an ultra-shallow n+ layer to minimize the Schottky barrier in Si in a FM/I/Si contact, where I is an ultra-thin thermal SiO2 or SiON. TEM analysis reveals time-dependent thickening of these barriers with a negative impact on the contact resistance. Temperature-dependent transport characterization indicates that the Schottky barrier is alleviated somewhat. I-V shows contacts that are still too resistive due to the reesidual Schottky barrier plus the thickened SiO2/SiON. We now insert an inter-layer of a low work-function rare-earth ferromagnet (Gd) to reduce the metal-semiconductor work-function mismatch and reduce the Schottky barrier further. This is seen to reduce the contact resistance by more than an order of magnitude and make it optimal for a wide range of channel dopings. Further, this technique reduced the Gd film thickness requirement with respect to a Gd-only approach which should result in higher spin polarization. Secondly, we demonstrate non-ohmic but optimally low contact resistances for CoFe contacts on crystalline Ge3N4 on Ge. This suggests the possibility of using other, possibly lower band-gap, insulators to achieve conductive spin contacts by a combination of Fermi-level de-pinning and charge dipole formation.
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