IFMGEDMS · Investigation of Ferromagnetism inGe-Based Diluted Magnetic Semiconductors

Investigation of Ferromagnetism inGe-Based Diluted Magnetic Semiconductors

The proposed research project addresses main issues in the field of ferromagnetic semiconductors which are candidates for use in low power consumption magneto-electronic devices. Within the scope of this project we combined experimental expertise in epitaxial growth, structural, magnetic and electronic structure investigation of ferromagnetic semiconductors to understand the origin of ferromagnetism and contributed to the development of fully operational semiconductor spintronic devices. The goal of this project is to synthesize thin films of Mn-doped Ge systems with magnetic order up to room temperature. In this project, molecular beam epitaxy (MBE) technique is used to fabricate Mn doped Ge DMS in order to incorporate Mn into Ge beyond its solubility limit. In the course of the project, a detailed structural and magnetic characterization of Mn doped Ge films synthesized under various growth conditions is carried out and the dependence of the magnetic properties of the films on the composition and growth parameters is investigated. This enables us to optimize the material properties of MnxGe1-x thin films to achieve strong magnetization and high Curie temperature which is the main objective of the project. In order to synthesize and characterize these materials, a custom made MBE system integrated with x-ray photoelectron spectroscopy (XPS) was built by our group. Combination of magnetic characterization and spectroscopic techniques were employed to find optimal conditions to incorporate highest concentrations of diluted Mn, to determine the bonding sites of Mn, and to investigate the origin of ferromagnetism. We have grown Mn doped Ge thin films on Si wafers by alternating deposition of atomic-scale layers of Mn and MnGe to carefully control the distribution of Mn atoms in Ge. The diffusion range of Mn atoms in Ge spacer layers can be adjusted by varying the substrate temperature. It was found that the magnetic ordering and Curie temperature could be controlled by adjusting the distance between the Mn ions in the Ge spacer layer depending on growth temperature. The samples with 4% Mn grown below 130 ̊C contained highly disordered ferromagnetic Mn rich domains with Curie temperature (Tc) of 150 K. Both structural and magnetic characterizations revealed that Mn5Ge3 precipitates were formed in Mn clusters free Ge matrix when the samples were grown at 150 ̊C and above. Furthermore, we have tried to grow these materials under nitrogen partial pressure to provide easy incorporation of Mn in the lattice and enhance the ferromagnetism. It is observed that introduction of N2 during growth effects the distribution of Mn atoms in Ge and leads to improved crystallinity and magnetic properties when compared to vacuum deposited films. In the presence of nitrogen, Tc increased upto 190K due to the increase in the number of active Mn atoms, however promoted Mn diffusion led to formation of Mn5Ge3 clusters. Although numerous efforts have been devoted to increase the Tc of Ge based DMS materials, Curie temperatures of grown MnxGe1−x alloys are lower than those measured for the Mn5Ge3 thin epitaxial layers. Due to high Curie temperature and sufficiently high spin polarization, we have focused our attention to the synthesis of thin films of Mn5Ge3 on Si to allow it to be directly incorporated into Si based spintronic devices. We have investigated the formation of epitaxial Mn5Ge3 films on Si substrates using solid phase epitaxial growth with the introduction of a thin Ge buffer layer. Then we investigated the influence of Ge layer thickness on the formation of epitaxial Mn5Ge3 films on Si substrates and their magnetic properties and thus establish the controbility of the magnetization. At the end of the DMS project, we have gained significant expertise on fabrication of MnxGe1-x diluted systems and Mn5Ge3 phase directly on Si with controlled magnetic properties. DMS project helped Dr. Leyla Colakerol Arslan establish her research group and set-up her laboratory. Overall one M.S. thesis is completed and three Ph.D. students are still continuing their studies under her supervision at Gebze Technical University (GTU) with full or partial support from this project. She also established collaboration in Turkey, Germany and USA. Several publications, conference contributions were made during the project for general dissemination of the results obtained.

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