Evidence for the Presence of Spin Accumulation in Localized States at Ferromagnet-Silicon Interfaces

We experimentally show evidence for the presence of spin accumulation in localized states at ferromagnet-silicon interfaces, detected by electrical Hanle effect measurements in CoFe/n + -Si/n-Si lateral devices. By controlling the measurement temperature, we can clearly observe marked changes in the spin-accumulation signals at low temperatures, at which the electron transport across the interface changes from the direct tunneling to the two-step one via the localized states. We discuss in detail the difference in the spin accumulation between in the Si channel and in the localized states. PACS numbers: The injection of spin-polarized electrons, manipulation of the injected spins, and detection of the manipulated spins were achieved for semiconductor (SC) devices.[1– 3] Thanks to these technological jumps, many advanced physical phenomena in a field of semiconductor spintronics have so far been revealed.[4–8] Recently, the electrical detection of spin accumulation and its depolarization in SCs through the Hanle effect in the three-terminal lateral devices has frequently been reported for GaAs,[9, 10] Si,[11–17] and Ge.[18–21] Since the device geometry used in this measurement is very simple and the multiple ferromagnetic contacts or submicron-sized fabrication processes are not necessary, this method is usually utilized as an evidence for the first step of the spin injection and detection in SCs.[9–21] However, underlying physics of the spin-accumulation signals detected in the three-terminal methods has been argued. The critical issue was firstly raised by Tran et al. in Fe/Al2O3/GaAs structures.[10] They reported a considerably large voltage drop (|�VHanle|), i.e., the magnitude of spin accumulation, of � 1 mV, which cannot be explained by the theory based on spin injection and spin diffusion.[22–25] In general, since the spin accumulation in SCs is corresponding to the difference in electrochemical potential between up and down spins, such large |�VHanle| cannot be created in the large density of states in the SC channels.[22–25] Eventually, they theoretically claimed that the large |�VHanle| originates from the spin accumulation in extrinsic localized states such as ionized donors within the depletion layer or surface states at the insulator (I)/SC interface.[10] Very recently, Jansen et al. also explained theoretically the magnitude of spin accumulation created in SC channels by spin-polarized tunneling via interface states.[26] Their study particularly showed an influence of the parallel event of direct tunneling and two-step one via the localized states on