Modeling of Spin Injection and Spin Transport Properties in Organic and Inorganic Semiconductors
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Modeling of Spin Injection and Spin Transport Properties in Organic and Inorganic Semiconductors P. P. Ruden,1 J. D. Albrecht,2 and D. L. Smith3 1 Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, U.S.A. 2 Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, U.S.A. 3 Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. ABSTRACT Spin polarized charge carrier injection and transport in non-magnetic semiconductors is a key enabling mechanism for spin based electronic data processing. We present theoretical models to describe spin injection and spin transport in structures consisting of a ferromagnetic metal injector, a thin semiconductor layer, and a ferromagnetic metal collector. The semiconductors considered are conjugated polymers (e.g. PPV), small-molecule organic molecular crystals (e.g. pentacene), and inorganic semiconductors (e.g. silicon). In thermal equilibrium the charge carriers in these semiconductors are not spin polarized. Efficient spin injection requires that the semiconductor be driven far out of local thermal equilibrium. Since carrier mobilities (and other relevant parameters) in polymers, organic molecular crystals, and inorganic semiconductors differ by many orders of magnitude, their charge carrier injection characteristics differ significantly. INTRODUCTION Motivated by the possibility of using the spin of mobile charge carriers as the physical entity representing information, recent electronic materials research has focused on physical phenomena such as spin polarized electron injection and spin polarized transport [1]. Several experiments have successfully demonstrated spin injection from ferromagnetic metals into conventional (inorganic) semiconductors [2,3,4]. In most cases the spin polarization was detected optically, but electrical detection based on ferromagnetic contacts in spin-valve devices is feasible and of greater practical relevance. Many of the structures investigated were rather complex, incorporating heterojunctions and carefully designed doping profiles. Very recently, spin injection into organic semiconductors and electrical detection of the spin current have also been reported [5,6]. Control over doping in organic semiconductors is very limited, hence these materials are usually undoped and essentially all mobile charge carriers are injected from the metal contacts. Electronic devices may be fabricated on films of polymers, films of small organic molecules, or on single molecular crystals of (small) organic molecules. Typical mobilities in these types of materials vary over a wide range from very low in the case of polymers to comparable to amorphous silicon for organic molecular crystals. Charge carrier mobilities in polymers usually are strongly dependent on the applied electric field. A typical range of values for polymers such as PPV is 10-7cm2/V·s to 10-3cm2/V·s. Films of polycrystalline pentacene have mobility values on the order of 10-3cm2/V·s to 10-1cm2/V·s, and the characterization
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