Spin Valve Effect in Monolayer Transition-Metal Dichalcogenides
thesisposted on 2018-11-28, 00:00 authored by Bo Hsu
Two-dimensional semiconductors transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se) are prominent candidate materials for advanced devices in a variety areas such as electronics, optoelectronics, and spintronics. In this dissertation, the growth and characterization of high-quality large-scale single-crystalline MX2 monolayers are discussed first; then the fabrication of spin valves with MX2 monolayer as the tunnel layer are presented, followed by the characterization and results discussion of these MX2 spin valve devices at various temperatures; finally the flexible MX2 spin valves were fabricated, characterized, and discussed. In Chapter 1, an introduction to spintronics and 2D materials is given, in particular, the physics of spin valves, background of 2D TMDC MX2, and an historical view of the development of classical spin valves with magnetic tunneling junctions based on MgO and Al-O spacer layers were discussed. A a brief literature review section of the most recently reported spin valves with 2D materials as spacer layers was given. In Chapter 2, large-scale single-crystalline monolayer MX2 were grown using chemical vapor deposition on SiO2 coated Si substrates. . The growth of MX2 is optimized by tuning the growth temperature, carrier gas composition and flow rate, precursor ratios, the local vapor pressure near the substrates, and growth time. The largest monolayers MX2 achieved are ~190 μm, ~450 μm, ~80 μm, and ~180 μm for MoS2, WS2, MoSe2, and WSe2 respectively. In Chapter 3, the structural, optical, and electrical characterizations of MX2 are reported and discussed including optical microscopy, scanning electron microscopy, atomic force microscopy, Raman scattering, photoluminescence (PL) spectroscopy, field-effect transistor transport measurements, and Hall effect measurements. In Chapter 4, vertical spin valves with MX2 (M=Mo,W, X=S,Se; four types of materials in total) monolayers as spacer layers and Co and NiFe as top and bottom ferromagnetic layers were fabricated on SiO2-coated Si substrates using wet-transfer approach and photolithography. Tunneling magnetoresistance (TMR) of MX2 spin valves were measured from 15 K to room temperature. In MoS2 spin valves, TMR ratios are up to 0.60% at room temperature and up to 0.83% at 16K. The TMR ratios of WS2 spin valves are up to 0.72% at room temperature and up to 1.42% at 16K. The TMR ratios of MoSe2 spin valves are up to 0.30% at room temperature and up to 0.38% at 16K. The TMR ratios of WSe2 spin valves are up to 0.19% at room temperature and up to 0.40% at 16K. Additionally, various properties of MX2 spin valves, including annealing effect, exchanged-bias effect, thickness dependence, magnetic anisotropy, and “aging effect” were also carefully studied systematically on a large number of devices. In Chapter 5, flexible spin valves with MS2 (M=Mo and W; two types of materials in total) monolayer as spacer layer and Co and NiFe as top and bottom ferromagnetic layers were fabricated on flexible polyimide substrate using wet transfer and photolithography processes. The devices show superior performance with small TMR degradation over a large range of bending of the substrate. The room-temperature TMR ratios for the first MoS2 flexible spin valve were 0.20 ± 0.01 % without bending, and 0.15 ± 0.01 % at 1 cm bending radius. In the second MoS2 flexible spin valve, TMR ratio of 0.27 ± 0.03 % and 0.14 ± 0.01 % were observed at no bending and at 1 cm bend radius, respectively. The room-temperature TMR of WS2 flexible spin valve were 0.37 ± 0.01 % without bending and 0.32 ± 0.02 % at 1 cm bending radius.