TOWARDS ORGANIC SPINTRONICS: TUNING THE CARRIER INJECTION BARRIER AT THE ORGANIC/FERROMAGNET INTERFACE

Abstract

The discovery of giant magnetoresistance (GMR) is considered to be the beginning of a new era of spin-based electronics. The spin valve, one basic unit of spintronic devices, consists of two or more conducting ferromagnetic layers with different hysteresis curves and one sandwiched nonmagnetic layer (conductor/semiconductor). The application of p-conjugated organic molecules as spin transporting layers in spin valves has attracted tremendous research interest. Due to their extremely weak spin-orbital and hyperfine interactions, exceptionally long spin diffusion lengths can be expected for organic semiconductors, making them particularly suitable for the transport of the spin polarization. For organic semiconductor based spin valve devices, ¿interfaces are critical¿. The interface between a ferromagnetic metal electrode and organic transporting layer is of crucial importance for spin transfer across the interface. By modifying the interface, the spin polarization can be controlled, for example, via the insertion of a buffer layer (e.g. MoO3 of high work function or Alq3 molecules of low work function). To successfully engineer the interface between ferromagnetic metals and organic semiconductors, it is necessary to obtain a comprehensive understanding of its electronic and morphological properties. Although interfaces between organic materials and normal metals have been extensively investigated, the published literature on the interfaces between organic semiconductors and ferromagnetic metals is limited. A detailed and systematic study of the interface between various organic semiconductors and ferromagnetic metal substrates is timely. This thesis aims to study the interfacial electronic properties and energy level alignments between various p-conjugated organic semiconductors and ferromagnetic metal Co, and to optimize the interfacial carrier injection barrier by introducing a buffer layer (i.e. MoO3 and Alq3) at the interface. In Chapter 2, the experimental tools including PES and AFM will be introduced. Chapter 3 describes the energy level alignments at the interfaces between four different organic molecules (i.e. CuPc, pentacene, C60 and PTCDA) and the ferromagnetic Co substrate, and to characterize the morphological properties of organic thin films on Co substrate, which serves as a foundation for the subsequent chapters. In Chapter 4, I investigate the electronic structures and properties at the interface between MoO3 and Co, and clarify how MoO3 buffer layer modifies the electronic structures of Co substrate. In chapter 5, the energy level alignments between Co and p-type organic molecules (CuPc and pentacene) after the insertion of MoO3 buffer layer are compared with the results in Chapter 3, to address how the MoO3 buffer layer tunes the hole injection barrier ¿h at CuPc/Co and pentacene/Co interface. In Chapter 6, I examine the interfacial electronic structures between Co and Alq3, and compare the energy level alignments between Co and n-type organic molecules (C60 and PTCDA) after the insertion of Alq3 buffer layer with the results in Chapter 3, and examine how the Alq3 buffer layer affects the interfacial energy level alignment and tunes the electron injection barrier ¿e at C60/Co and PTCDA/Co interface. Chapter 7 presents the summary and outlook of this study.