Browsing by Author "Wang, Tao"
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Item Fabrication of Germanium Tin Microstructures Through Inductively Coupled Plasma Dry Etching(IEEE Transactions on Nanotechnology, 2021-09-30) Lin, Guangyang; Cui, Peng; Wang, Tao; Hickey, Ryan; Zhang, Jie; Zhao, Haochen; Kolodzey, James; Zeng, YupingGermanium tin (GeSn) with a Sn content of >12% has a great potential for optoelectronic devices due to its direct bandgap property. In this work, the anisotropic etching of GeSn with Sn content of 12.5% and selective etching of Ge over GeSn were explored by inductively couple plasma (ICP) dry etching to obtain various microstructures. Through adding oxygen into chlorine and argon and adjusting the process pressure, the anisotropic etching of GeSn was optimized with an ideal sidewall angle of 89 o . The optimized process is compatible with both positive and negative resists. By altering the ICP power, Ge etching recipes with low and high etching rates were developed, which are favorable for fabricating GeSn nano- and micro-structures, respectively. An etching selectivity of >126 for Ge over GeSn with Sn content of >10% can be achieved. With the optimized dry etching recipes, suspended GeSn microribbons and microdisks were realized. Ultimately, the suspended GeSn microstructures were transferred onto 40-nm-thick ZrO 2 on p + -Si to form a GeSn-on-insulator (GeSnOI) substrate. For a fabricated 45-nm-thick Ge 0.875 Sn 0.125 OI back-gated transistor, the subthreshold swing (SS) of 240 mV/dec is reasonably low for a non-optimized device, suggesting that the explored dry etching methods are promising for device processing.Item Second-harmonic signature of chiral spin structures in W/Pt/Co heterostructures with tunable magnetic anisotropy(Journal of Physics D: Applied Physics, 2023-03-28) Wang, Yang; Chan, Ying-Ting; Wang, Xiao; Wang, Tao; Cheng, Xuemei M.; Wu, Weida; Xiao, John Q.Second-harmonic Hall voltage (SHV) measurement method has been widely used to characterize the strengths of spin–orbit torques (SOTs) in heavy metal/ferromagnet thin films saturated in the single-domain regime. Here, we show that the magnetic anisotropy of a W/Pt/Co trilayer can be robustly tuned from in-plane to out-of-plane by varying W, Pt, or Co thicknesses. Moreover, in samples with easy-cone anisotropy, SHV measurements exhibit anomalous 'humps' in the multidomain regime accessed by applying a nearly out-of-plane external magnetic field. These hump features can only be explained as a result of the formation of Néel-type domain walls, efficiently driven by nevertheless small SOTs in this double heavy metal heterostructure with canceling spin Hall angles.Item Spin orbit coupling induced phenomena in spintronic devices(University of Delaware, 2017) Wang, TaoSpin-orbit coupling (SOC) is a relativistic interaction between spin and orbital momentum that induces a plenty of novel phenomena including spin Hall effect (SHE), Rashba-Edelstein effect (REE), topological surface states (TSS), spin orbit toques (SOTs), etc. In this thesis work, we have conducted relevant researches in order to develop materials and heterostructures, as well as new phenomena that can lead to high efficiency in generating SOTs or switching the magnetization. We have also investigated fabrication technique that speeds up the fabrication of nanoscale devices. ☐ A nanofabrication procedure has been developed based on shadow mask and angle deposition techniques. It is an efficient bottom-up, as oppose to top-down lithography technique, to fabricate rather complicate devices. A comprehensive study of shadow mask technique has been accomplished to fabricate magnetoelectric (ME) device based on voltage controlled boundary magnetism of antiferromagnet Cr2O3 and magnetic tunnel junctions. ☐ The researches on SOTs focus on SHE, REE and TSS in which a charge current is converted into spin current that exerts the SOTs on the magnetization of neighboring ferromagnetic layer. First, we have studied the charge to spin conversion efficiency in 3d light transition metal vanadium in V/CoFeB bilayer, which shows a surprisingly large spin Hall angle that is comparable to that of Pt. Second, we have investigated the temperature dependent SOTs in Bi2Se3/Co and Bi2Se3/Ni80Fe20 bilayers, and confirmed the existence of TSS in Bi2Se3/Co systems. Finally, we have discovered a novel spin rotation behaviors in a ferromagnetic metal, where the spin polarization is rotated around the magnetization. It implies that a perpendicularly polarized spin current can be generated by an in-plane charge current through a ferromagnetic metal, which can be applied to realize the anti-damping switching process in magnetic heterostructures.