Projects

Spin-orbit torque in fully amorphous iron silicide

Material system

Fully amorphous iron silicon solid solution. Fully amorphous structure is characterized via synchrotron X-ray grazing incidence XRD at SSRL in SLAC. Lack of short-range ordering is characterized by high-resolution cross-sectional TEM. Iron silicide is deposited at room temperature via co-sputtering.

Spin-Orbit Torque

Strong Fe concentration dependence of SOT shows a non-monotonic trend with increasing Fe content (resistivity). Large SOT efficiency rivaling topological insulators is discovered near 45 % Fe. 

SOT switching of PMA Magnet

Efficient switching of magnetization with perpendicular magnetic anisotropy can be achieved with critical current density of same order as topological insulator and sputtered chalcogenides. 

Spin-orbit torque in fully amorphous cobalt silicide

Material system

Co-sputtered Co and Si target was used to deposit amorphous cobalt silicide at room temperature. The amorphous structure is confirmed with high-resolution cross-sectional TEM similar to iron silicide. The magnetic transition point of Cobalt silicide is sharp with the transition critical concentration to be around 68~70% Co.

Spin-Orbit Torque

Co concentration dependence of spin-orbit torque manifests as a sharp peak profile around 68% Co with the SOT drops sharply to both side. Interestingly, the onset of ferromagnetism is near 68% characterized through XMCD at ALS in LBL. 

Enhanced Spin Hall conductivity

With larger SOT found at high Co content Cobalt silicide, the resistivity value is relatively low thus boosting the spin Hall conductivity which is the product of conductivity and SOT efficiency (i.e. spin-charge conversion efficiency).

Tunable magnetic canting in ferrimagnetic GdCo                                

Material system

Co-sputtered Gd and Co element is done under room temperature to create GdCo solid solution near the magnetic compensation concentration. When GdCo is deposited near the compensation concentration, magnetic anisotropy canting can be found to be away from the out-of-plane direction with proportions of in-plane anisotropy creating a canted easy-axis. In addition, there exists a two-phase structure with the first few angstrom to a nanometer of GdCo to be much dense thus yielding a larger coercive field. This is an ideal system for engineering exchange spring systems. 

Canted magnetic anisotropy

Easy-axis with angles ranging from 13 to 56 degrees away from the out-of-plane direction can be obtained. 

Field free SOT switching/Multistate switching / Ratchet effect / Memristor

[Field free] Since the anisotropy is away from the out-of-plane direction, field-free switching can be easily achieved. Though observation of clean switching diminishes as the canting angle increases since anomalous Hall effect is direct proportional to amount of out-of-plane magnetization. Nevertheless, field-free switching is achievable regardless of the canting angle.  

[Multistate] Due to the two-phase magnetic system where first few angstroms of GdCo is more dense, the exchange spring effect and the anisotropy canting induces multistage switching in such canted GdCo magnetizations.

[SOT Ratchet] Due to the same exchange spring and canting effect, ratchet effect is dominant where asymmetry in SOT switching can be observed across chirality and polarity.

[Memristor] Due to the exchange spring and canting effect, the domain walls can be stabilized and easily moved with memory corresponding to the switching current amplitude. 

Ferroelectric - antiferroelectric thin film heterostructure for negative capacitance transistor

Material system

Atomic layer deposition of fluorite hafnium and zirconium oxides are carried out on silicon wafers with wet-processed chemical oxides. Superlattice stacking of mix-phased hafnia and zirconia with their thicknesses and phases optimized for capacitance enhancement following the negative capacitance design. 

Reduced EOT approcahing the chem-ox thickness

By following the negative capacitance design principle, stacking of antiferrfoelectic and ferroelectric thin films stabilizes the flattened energy landscape. This leads to an enhanced permittivity which lowers the EOT of this gate dielectric and boosts the overall gate oxide capacitance when integrated into MOSCAPs. The EOT of such stack is approaching the 6.5 angstrom chemically grown oxide on top of the silicon wafer. 

Reduced EOT without degrading channel mobility 

Conventionally, lowering of EOT for better gate control and device performance is challenging without sacrificing channel mobility. The conventional method to lower the EOT is to scavenge the thin chemical oxide below the high-k dielectric. However, by doing so, the overall gate oxide thickness is reduced leading to a higher gate leakage and stronger remote phonon scattering due to the gate electric field. In this superlattice gate stack, the scavenging of chemical oxide interfacial layer is not necessary to achieve a low EOT due to the negative capacitance design. This means that without sacrificing channel mobility, a lower EOT thus a better transistor performance can be achieved.