The Fabrication Of Molecular Semiconductor Films With Anisotropy And Investigation Of Spintronic Devices

Organic spin valve(OSV)is one of the most important research objects in organic spintronics.Significant progress has been made in understanding the spin injection,manipulation and detection of OS Vs since its discovery.However,the research in this field is still in its infancy and there are still many limits for commerical application.Generally,the magnetoresistance(MR)effects of spin valve have strong temperature dependence and tend to decrease sharply with the increase of measurement temperature.What's more,how to improve the quality of the electrode/organic interface is also a challenge for scientists in this research field.To overcome these limitations,people should seek the appropriate organic materials,which are suitable for spin valve devices.In addition,people need to deeply explore the structure of organic thin film layer,spin electron transport properties,properties of ferromagnetic electrode/organic interface and so on.Based on the research,scientists try to getting an insight on the material factors,which influence spin transport behaviors of spin valve devices and then to improve the device performance of OSVs.It can also help us to explore the transport mechanism of the spin electron in organic semiconductor materials.In this work,the fullerene-based OSVs using C60 and C70 molecules were investigated.Among various organic semiconductor candidates,fullerenes(C60 and C70)have been extensively investigated for their abundant properties in electricity,catalysts,magnetism and optical transition.Owing to the lack of hydrogen element,the fullerenes molecules should possess much weaker hyperfine interaction than that of other organic semiconductors with hydrogen.C60 molecule caught more attentions in spintronics due to the successful operation at room temperature and morphology effect of C60 film on MR ratio was investigated.As an important member of fullerene family,C70 molecule can be envisioned by adding a ring of five hexagons along the equatorial plane of C60.So C70 is also an electron acceptor and could be used for spintronics.We also find that the crystal structures of the organic films applied in organic spin devices reported so far are mainly amorphous.As the molecules in amorphous organic films are in disorder,the electron transfer efficiency is limited,which will hinder the injection of the carriers.Amorphous organic layers used in organic spin devices will cause serious conductivity mismatch at the organic/magnetic electrode interface.This will give rise to electron scattering at the interface.Moreover,the barrier caused by disordered arrangement of the amorphous distribution and a large number of defects will increase the spin scattering of electron in the organic layer.As a result,it will hinder the electron transmission in the devices.In contrast,the single crystal organic thin films have high-order,high mobility and so on.Therefore,we carry out further study about the crystal growth of copper phthalocyanine(CuPc)film.We try to find out the influences on the stability of CuPc film caused by the temperature of substrate.The research results are summarized as follow:1.We have fabricated C60 and C70 OSVs with an MgO substrate/F e3O4/Al-O/fullerenes/C o/Al stacking structure.The films were characterized by atomic force microscopy(AFM),Raman spectroscopy,physical property measurement system(PPMS)and so on.We inserted the Al-O layer between the electrodes Fe3O4 and Co,which effectively reduce the electron transport barrier between the electrode and the organic layer.As a result,the electron transport efficiency was greatly improved.Tunneling MR effect was found in both fullerene spin devices,and still exists till room temperature.So fullerenes based spin devices are excellent candidates to investigate the spin transport behaviors in organic semiconductors.2.It is found that the MR ratio at 300 K was approximately 3%for the C60 based devices.However,MR ratio is only 0.3%for the C70 based device at 300 K.This means the MR ratios of C70 based device is one order of magnitude smaller than that in C60 based device at 300 K.We found that the smaller MR ratios in C70 spin valves likely originated from the anisotropy of C70 layer.This study is valuable for future organic material design in spintronics operated at room temperature.3.As phthalocyanine compounds have shown significant advantages in previous semiconductor devices,we hope to introduce CuPc as an organic spacer into the spin valve device.It is known that CuPc exists in different polymorphic forms,including ?,?,? and so on.So it is vital to obtain CuPc film with pure phase to be applied in spin devices.A series of peaks were observed in the XRD patterns of CuPc powder,which was a typical character of polycrystalline structure.After optimizing the growth conditions of CuPc films,we fabricated the CuPc films with a high texture along(200)orientation of a phase.We found the crystalline of CuPc films didn't change even with the substrate temperature up to 200?.The crystal films with high orientation provide an excellent channel for spin conserved electron transport in the spin valve.Furthermore,due to the high thermal stability of the films,the spin valve device with CuPc is expected to achieve an excellent MR effect at room temperature.