|Author:||Wong, Wang Cheung|
|Title:||Spintronic devices based on metal dichalcogenides materials|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Applied Physics|
|Pages:||78 pages : color illustrations|
|Abstract:||Two dimensional (2D) materials such as graphene or MoS₂ are well-suited for use in electronic devices. For example, the high on-off ratio of transistors based on 2D materials has drawn much attention as the replacement of silicon transistors. Besides, graphene also demonstrates outstanding spintronic properties, with long spin lifetime (1-6 ns at 4 K) and spin diffusion length (3-12 μm at 300 K) as compared with conventional semiconductors (GaAs, highly doped Ge) or metals (Cu, Al). As a result, graphene-based spin logic devices have been proposed. Compared with graphene, MoS₂ is a 2D material that is more often investigated for optoelectronics applications. At the same time, MoS₂ also shows special spin-related properties. Due to the symmetry breaking that exists in thin MoS₂, a long spin lifetime in the order of nanoseconds was predicted. Fe/MoS₂/Fe magnetic tunnel junctions were predicted to demonstrate large magnetoresistance ratio (MR) of 300% according to fully atomistic first-principle transport calculations, which was attributed to an efficient spin injection due to the strong hybridization between iron and sulphur atoms. Moreover, planar (edge-contacted) Fe/MoS₂/Fe junctions were estimated to yield MR of 150% using non-equilibrium Green's function calculations. Experimental results of NiFe/MoS₂/NiFe junctions, however, showed a mere MR of 0.73% and was significantly different from the calculation results. Typically, the MoS₂ spacers in tunnel junctions were fabricated by wet transfer ex situ, which may trap contaminants or induce inhomogenous coverage of electrodes surfaces, and could also lead to oxidation of electrodes during the transfer process. These unwanted artifacts could decrease the quality of contact interfaces. Previous investigations on boron nitride magnetic tunnel junctions with the spacer layer grown in situ by chemical vapour deposition (CVD) showed an order of magnitude improvement in the MR compared with wet-transferred spacers, highlighting the importance of the contact quality on the junction performance.|
In this thesis, I fabricated La₀.₇Sr₀.₃MnO₃ (LSMO)/MoS₂/NiFe vertical spin valves, with the MoS₂ spacer layer prepared by magnetron sputtering. The choice of bottom electrode is based on the chemical stability of LSMO against reaction with oxygen. Sputtering process eliminates contaminants and incomplete coverage of bottom electrodes by MoS₂, thus improving the quality of contacts between MoS₂ and the magnetic electrodes. In this project, four-point electrical measurements were used to measure both electrical and transport properties of the devices. LSMO bottom electrodes were deposited by pulsed laser deposition (PLD) on SrTiO₃ (STO) substrates. MoS₂ thin films were deposited on LSMO by direct RF magnetron sputtering at room temperature to suppress sulphur vaporization. The MoS2 layer was then post-annealed at 450°C under a nitrogen atmosphere of 1 atm. by rapid thermal annealing to enhance the crystallinity. NiFe top electrode and Au capping layer were finally deposited by electron-beam evaporation. MoS₂ films sputter-deposited on LSMO showed characteristic E₂g and A₁g vibrations of MoS₂ in Raman spectroscopy. The result suggested that crystalline MoS₂ was successfully grown on LSMO. Using this sputtering method, MoS₂ films could be grown on various substrates at lower temperatures as compared with CVD. MR measurement of such MoS₂-spacer spin valve showed 0.8% MR at 20 K. The results suggested a method to fabricate 2D material-based spintronic and electronic devices with reliable contacts.
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