| In the 20th century, devices and integrated circuits based on the electroniccharges and their transportation have been widely used in the world. However, theelectron is a quantum mechanical object which, apart from charge, also has a spin.The electron is treated as the carrier of the charge and the spin of the electron usuallyare neglected. In 1988，the discovery of GMR (giant magnetoresistance) and TMR(tunneling magnetoresistance) in metallic spin valves have revolutionized applicationssuch as magnetic recording and memory, and launched a new field of spinelectronics—’spintronics’, which is centered on the electron spin including theirgeneration, transport and detection. Electron spin injection and spin dependenttransport are essential aspects of spintronics and have been extensively studied in anumber of different contexts including: from ferromagnetic metals to superconductors;from ferromagnetic metals to normal metals; from ferromagnetic metals tononmagnetic semiconductors and from magnetic semiconductors to nonmagneticsemiconductors or the mixture of them.Compared with conventional semiconductors, organic semiconductors haveabundant electronic, magnetic and optics characteristic and are much more easilysynthesized at low temperature with little toxic waste. Based on the electronic andoptical properties of organic semiconductors, light-emitting diodes for flat-screen TVs,cell phone displays, billboards and computer display screens have been fabricated. Asthe spin-orbit coupling is weak, organic semiconductors are potential candidates forspintronic applications. Spin polarized injection and transport in organicsemiconductors is a interesting topic in spintronics. In recent years, the study ofspintronics in organic semiconductors has obtained certain research results, and itbecame a new subject line branch-organic spintronics.In experimental studies, in 2002, Dediu et al. observed spin injection into thinfilms of conjugated organic material sexithienyl (T6) in the sandwich structure of La0.7Sr0.3MnO3/T6/La0.7Sr0.3MnO3(LSMO/T6/LSMO) at room temperature. In 2004,Xiong et al. built a spin valve using the 8-hydroxyquinoline-aluminum Alq3as aspacer, where the Alq3is sandwiched between layers of cobalt and half-metallicmanganite LSMO. In 2006, Majumdar et al. have observed the spin injection andtransport in the structure of LSMO/polymer/Co, the influence of the interfacial barrieris also discussed with emphasis. In 2008, Dediu’s group reported the spin polarizedinjection and transport in LSMO/Alq3/Al2O3/Co organic device. They employedvertical spin valve devices with a direct interface between the bottom manganiteelectrode and Alq3, while the top-electrode geometry consists of an insulating tunnelbarrier placed between the "soft" organic semiconductor and the top Co electrode. In2009, Zhang et al. reported the spin injection and transport in Alq3/Al2O3/Co anddiscuss the effect of Al2O3buffer layer. Spin polarized injection and transport inorganic semiconductors not only can broaden our understanding on the physical worldof the organic materials but also can have a substantial impact on the applications ofspintronics and bionomics.Quantum theory used by Xie and spin diffusion theory used by Ruden, Smith,and Z. G. Yu have been demonstrated successfully for studying the spin polarizedinjection and transport in organic semiconductors. Quantum theory can be used tounderstanding the microstructure of ferromagnetic/organic system and the dynamicsof spin transport, the classic diffusion theory can be used to give the spin polarizedcurrent in organic semiconductor structures by integrating Ohm’s law.In this thesis, based on the classical model of one-dimensionalferromagnetic/organic semiconductor structure, a T-shaped organic semiconductordevice is designed. Based on the equation of spin drift-diffusion theory and Ohm’slaw, and also considering the relationship between charge–spin of the special carriersand interfacial effects in organic semiconductors, we have done some detailed studyon the current spin polarization and its amplification in the T shaped organicsemiconductor device. The results reveal that amplification of current spinpolarization under certain condition can be obtained in the T-shaped organicsemiconductor devices. Therefore, we call it organic spin transistor. The detailed research and main results are given below:1. Effect of polarons and bipolarons on the current spin polarization and itsamplificationDue to strong electron-lattice coupling in organic semiconductors，injectedelectrons can induce the distortion of the lattice and result in charged self-trappedstates called spin polarons or spinless bipolarons. Under the influence of externalconditions, such as temperature, pressure and external electric field, the polaron andbipolaron can be transformed into each other. And the nature of the material can affectthe polaron and bipolaron, the number and proportion of the produced polaron andbipolaron are discrepant for different materials. In this thesis, we assume that thepolaron in organic semiconductor exist at a certain proportion, without consideringthe decay of polaron and the transformation between the polaron and bipolarons, wehave calculated the change of current spin polarization when the polaron ratiochanged from 0 to 1. The results indicate that there is a direct relationship between thecurrent spin polarization and the ratio of polarons, the greater the ratio of polarons, thebigger the current spin polarization. Therefore, the material which is easy to producepolaron is more conducive to the spin polarized injection. Suitable ratio of polaroncan achieve effective amplification of the current spin polarization.2. Effect of electric field on the current spin polarization and its amplificationThe spin diffusion length represents the strength of spin injection. It issusceptible to the influence of external conditions, such as temperature, external fieldand the external pressure. The current density of various branches in the T-shapeddevice is directly related to the electric field. The current spin polarization is alsoconnected with the spin-dependent current density. We considered the effection ofelectric field on the spin diffusion length, and then obtained the effect of the electricfield on the spin polarized current. Through the calculation, one can find that strongelectric field can improve the efficiency of spin injection, but it is not conducive to thespin polarization amplification in the T-shaped device.3. Effect of interfacial resistances on the current spin polarizationFrom the calculation, it is found that big current spin injection can be obtained by modulating the spin-related interfacial resistances. Interfacial resistances can beobtained through the tunneling barrier. Organic semiconductors have self-regulatoryfunction, so it can form relatively small interfacial resistances and they can bespin-related. The proposed amplification scheme provides an efficient way to generatea highly spin-polarized current in organic semiconductors because of the easilyfabrication and easily adjusting of ordered spin-related tunnel barriers at contactstructure. |
PDF Full Text Request