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This dissertation represents a set of work in the field of semiconductor spintronics. We address an important issue in this field that involves spin injection efficiency in spintronics devices from a ferromagnet into a non-magnetic semiconductor medium. The problem occurs at the interface of the ferromagnetic material, which is a source of spin-polarized electrons and a non-magnetic semiconductor. The spin injection efficiency is found to be low when the ferromagnet has much higher conductivity than the semiconductor. We have taken several steps to approach this problem. We grow and fabricate Europium Sulfide (EuS) as a source of spin polarized electrons with spin polarization close to 100%. The high spin polarization and semiconducting properties of this material are the two main reasons we chose EuS. In addition, EuS can be doped and hence its conductivity can be changed from insulating to highly conducting. As an insulator EuS can serve as a spin filter, and when conducting, it can be used as a source of spin polarized current and a sensitive detector of spin polarized electrons. We present three projects involving ferromagnetic EuS by making structures on various metallic substrates and on the two semiconducting substrates. The work on metallic substrates including Ag, Au, In and Al showed the presence of a Schottky barrier in the EuS. Al can also serve as an Ohmic contact to EuS when evaporated at elevated substrate temperatures, in the range from 60˚C-100˚C. Detailed measurements of the current–voltage (I-V) characteristics shows a clear shift in the mean Schottky barrier height of ~0.26±0.06 eV, near and below the Curie temperature (TC) of the EuS film. In an effort to make an all-semiconductor structure, we grow EuS on two III-V semiconductor substrates, GaAs and Ga0.7Al0.3As. We established the basic picture of the heterojunction band alignment in these structures, and we measure I-V characteristics across them. The detailed analysis of the I-V characteristics of EuS/GaAs heterojunction, in the temperature range between 5K-150K, confirms the presence of the EuS Schottky barrier. Furthermore, the analysis of the I-V characteristics of the current injected from EuS into GaAs across the heterojunction, yields a value for the Zeeman splitting of the EuS conduction band of (0.48±0.12)eV at 5K. The change in the barrier height at the heterojunction mimics the change of the spontaneous magnetization of EuS, i.e. it has Brillouin like characteristics with a TC of 17K. Utilizing the experimentally obtained values for the Zeeman splitting as input parameters, we analyze the I-V characteristics for unpolarized electrons injected from GaAs, to estimate the polarization detection efficiency as a function of bias and temperature below 30K. In addition, we have fabricated heterojunctions of EuS and Ga0.7Al0.3As with the ability to change the carrier density in Ga0.7Al0.3As in situ by utilizing the persistent photoconductivity effect. The importance of this experiment is in the ability to verify the existence and the dominance of the EuS Schottky barrier since its band structure is only slightly different from that of GaAs. Moreover, we have verified that the transport across this interface is invariant to the change of the carrier density in Ga0.7Al0.3As, which confirms the uniqueness and suitability of using these two interfaces for future spintronics devices.
A Dissertation Submitted to the Department of Physics in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy.
Includes bibliographical references.
Florida State University
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