Controlling and probing charge carrier spin polarization in semiconductors via electrical means is an attractive route towards the development of practical semiconductor spintronic devices, which are expected to have a strong impact on future information processing and storage technologies.
Artist impression of a spin transistor. Spin polarized carriers flow in the gated channel between the ferromagnetic source and drain. The magnetization orientation of the ferromagnets affects the current and introduces a non-volatile memory effect. A practical spin transistor has not yet been developed.
Considerable success has been obtained over the last number of years in the field of inorganic semiconductor spintronics, with the demonstration by our group of, for example, the creation and detection of a robust spin polarization in silicon at room temperature. For more information, see “Electrical creation of spin polarization in silicon at room temperature”, S.P. Dash, S. Sharma, R.S. Patel, M.P. de Jong, and R. Jansen
Detection of a robust spin accumulation in Si at room temperature. The spins of carriers injected into Si using a ferromagnet/oxide spin tunnel contact precess in an externally applied magnetic field: the electrical Hanle effect (top panel). For sufficiently strong fields, the precession frequency becomes comparable to the intrinsic spin-flip rate, which leads to a detectable suppression of the spin accumulation. The voltage drop over the spin tunnel contact scales with the spin accumulation in Si, hence this accumulation can be determined from the magnetic field dependence of the voltage.
Key to this success is the growing understanding of the physical mechanisms that govern the spin-dependent behavior of charge carriers in ferromagnetic-metal / semiconductor heterostructures, and in particular of the practical limitations for spin-polarized charge carrier injection and detection using ferromagnetic metal contacts. The latter is known as “conductivity mismatch”, which applies to the diffusive transport regime.
The new and promising field of organic semiconductor spintronics poses considerable challenges in this respect, as the physics underlying spin-polarized transport in organic devices remains somewhat elusive. Nevertheless, carbon-based, organic semiconductors offer a number of unique advantages. Spin lifetimes are potentially very long, bottom-up fabrication relying on self-assembly is possible, and non-stringent requirements for interface formation and film growth apply. The latter allows for, for example, the fabrication of vertical stacks comprising alternating layers of ferromagnetic metals and molecular semiconductors.
Three examples of carbon-based organic molecular semiconductors.
In the NanoElectronics group, we study several promising candidate systems for spintronics based on carbon-based materials and ferromagnetic metals for spintronics, which are outlined below. For a general introduction to the field, see “Organic Spintronics”, W.J.M. Naber, S. Faez and W.G. van der Wiel, J. Phys. D: Appl. Phys. 40 (2007) R205.
Vertical organic spin valves
Large magnetoresistance effects have been reported in vertical spin valves comprising thin film organic semiconductors, in several cases even at room temperature. It should be noted that a similar feat has not been obtained so far with inorganic semiconductors.
Left: Cross section and top view of a C60-based vertical organic spin valve fabricated by vapor deposition and shadow masking to obtain a cross-bar geometry. Right: magnetoresistance measurements of a spin valve comprising a 5 nm C60 layer.
For more information, see: “The multi-step tunneling analogue of conductivity mismatch in organic spin valves”, T.L.A. Tran, T.Q. Le, J.G.M. Sanderink, W.G. van der Wiel, and M. P. de Jong, Adv. Func. Mater. (2012).
It remains unclear, however, how spin-polarized charge transport takes place in these devices on a microscopic level. We study spin valves with ultra-thin organic semiconducting C60 layers, in which the transport is relatively easy to model, via multi-step tunneling processes. In this regime, charge carriers tunnel via a limited amount of steps involving localized intermediate states in the organic semiconductor. C60 is an attractive choice for spintronics: Since C60 molecules are purely composed of carbon, and the 99% 12C isotopes have zero nuclear spin, the effects of hyperfine fields on the spin polarization of charge carriers are very small.
The magnetoresistance as function of C60 thickness <10 nm is well described by a model that considers two-step tunneling processes. The squares/circles are measured data points, the dashed lines are model calculations.
It turns out that multi-step tunneling leads to a behavior analogous to conductivity mismatch in diffusive semiconductors like Si, in the sense that the inclusion of an increasing number of intermediate tunneling steps results in a more and more spin-independent junction resistance, regardless of the spin lifetime and spin diffusion length. This previously overlooked fact places numerous published studies on organic spin valves in an entirely new light, and provides new design rules for organic spintronic devices.
Vertical organic spin valves with engineered interfaces and comprising different organic semiconducting interlayers exhibiting different electrical properties are under ongoing investigation.
Lateral organic spin transport devices
Lateral device geometries are extremely useful for characterization of spin-dependent transport in semiconductors, using non-local techniques for detecting carrier spin polarization with ferromagnetic electrodes. Regarding organic semiconductors, quite strong constraints apply to the lateral transport geometries used, due to their intrinsically lower conductivity compared to inorganic semiconductors. We are developing such lateral structures for a variety of organic semiconductors.
Organic single crystals feature relatively high carrier mobility, and are thus attractive systems for lateral spintronic devices. We are working on devices consisting of single crystals that are laminated onto pre-patterned multi-terminal structures fabricated by e-beam lithography.
Lateral spin transport device structure prepared by e-beam lithography for definition of the contacts, and subsequent lamination of a rubrene single crystal. The right panel shows transistor characteristics for such a device. For more information see: “Controlled tunnel-coupled ferromagnetic electrodes for spin injection in organic single-crystal transistors”, W.J.M. Naber, M.F. Craciun, J.H.J. Lemmens, A.H. Arkenbout, T.T.M. Palstra, A.F. Morpurgo and W.G. van der Wiel, Organic Electronics 11, 743 (2010).
Carbon spintronics with graphene and graphite
Graphene is a highly promising material for spintronics due to its intrinsically weak spin-orbit coupling and hyperfine interaction, which translates into a long spin lifetime. Since graphene also exhibits an exceptionally high carrier mobility, spin polarized carriers travel over very long distances before the spin orientation is lost.
We study interfaces between graphene/graphite and ferromagnetic metals, which are essential for optimizing the injection and electrical read-out of spin-polarized carriers. As has been shown by our collaborators, the Computational Materials Science Group at MESA+, a perfect spin filtering due to k-vector conservation is possible in theory at epitaxial interfaces of Co (or Ni) and graphene/graphite. This is due to the fact that the only states available at the Fermi surface of graphene/graphite reside at the K-points, for which only minority spin states of the ferromagnets are present. We explore practical routes towards engineering such interfaces.
Atomic resolution STM image of Co adatoms adsorbed on graphite. The bright spots encircled in light pink refer to individual Co adatoms that were adsorbed on the b-site of the surface graphene layer, whereas those captured in light green correspond to Co tetramer formed by four Co adatoms. Three of these adatoms occupied the b-sites and the fourth adatom attached to either the a- or the overbond-site. The label H indicates the hollow site. For more information, see: “Growth mechanism and interface magnetic properties of Co nanostructures on graphite”, P. K. J. Wong, M. P. de Jong, L. Leonardus, M. H. Siekman and W. G. van der Wiel,
Phys. Rev. B 84 (2011) 054420.
Interfaces between organic semiconductors and ferromagnets
The microscopic mechanisms governing the magnetotransport behavior in organic spintronic devices remain poorly understood, in part due to the often ill-defined hybrid interfaces in the devices. This understanding may be improved upon exploiting the electronic structure and magnetic properties of well-defined interfaces between ferromagnetic electrodes and organic semiconductors.
We study such well defined interfaces, for example C60/bcc-Fe(001) for application in organic spin valves. X-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements show that hybridization between the frontier orbitals of C60 and continuum states of Fe leads to a significant magnetic polarization of C60 π*-derived orbitals. Both the magnitude and the sign of this polarization were found to depend markedly on the excitation energy, leading to oscillatory behavior near EF.
This demonstrates the possibility of controlling the interfacial spin polarization at well-defined hybrid interfaces via chemisorption, which is an essential ingredient for systematically engineering the performance of future organic and molecular spintronic devices. This is an ongoing theme in our organic spintronics research.
Top left: LEED pattern of an epitaxial bcc-Fe (001) film on MgO(001). Top right: XAS spectra of a multilayer (red) and monolayer (blue) of C60 on Fe (001);the broadening of the monolayer spectral features indicates orbital hybridization. Bottom: C K-edge and Fe L-edge (inset) XAS and XMCD spectra of a ML C60/Fe(001) sample. The Magnetic polarization of C60 orbitals is evident from the XMCD signal; this polarization changes sign upon changing the excitation energy for states near EF. For more information, see: “Hybridization-induced oscillatory magnetic polarization of C60-orbitals at the C60/Fe(001) interface”, T.L.A. Tran, P.K.J. Wong, M.P. de Jong, W. G. van der Wiel, Y. Q. Zhan, and M. Fahlman, Appl. Phys. Lett. 98 (2011) 222505.